A fluid delivery system configured for supplying fluid to a machine, comprising: a double diaphragm pump that includes a pneumatic mechanism mounted therein and provided with a first diaphragm, a second diaphragm, and an air-intake portion that brings air therein for driving the first and the second diaphragms (11, 12) to move and act through action paths, a first inlet and a first outlet that are disposed on one end of the pneumatic mechanism with the first diaphragm, a first fluid chamber formed between the first diaphragm, the first inlet and the first outlet, a second inlet and a second outlet that are disposed on another end of the pneumatic mechanism with the second diaphragm, a second fluid chamber formed between the second diaphragm, the second inlet and the second outlet, a plurality of check valves each of which disposed on the first inlet, the first outlet, the second inlet and the second outlet correspondingly; an introducing pipeline composed of a first guiding tube with one end thereof connected to the first inlet of the double diaphragm pump, and a second guiding tube with one end thereof connected to the first outlet of the double diaphragm pump; a return pipeline that includes a first return tube connected to the second inlet of the double diaphragm pump, and a second return tube connected to the second outlet of the double diaphragm pump; a fluid storage unit used for supplying fluid required and receiving fluid returned and connected to the first guiding tube and the second return tube; and a fluid supply portion that supplies fluid required to the machine and connected to the second guiding tube and the first return tube; wherein, in a first operation state, fluid in the fluid storage unit is passed through the introducing pipeline and the action path of the first diaphragm and delivered into the fluid supply portion to be used by the machine when the double diaphragm pump applies a pressure to the first diaphragm; fluid in the fluid supply portion is passed through the return pipeline and the action path of the second diaphragm, and returned to the fluid storage unit when the double diaphragm pump applies a pressure to the second diaphragm; characterized in that the fluid delivery system further comprises a drain-back tube arranged between the second guiding tube and the second return tube and provided with a control valve; wherein, when the control valve is closed, the fluid delivery system is in the first operation state; and wherein, when the control valve is open, the second guiding tube and the second return tube communicate with each other by the drain-back tube and the fluid delivery system is in a second operation state, in which the fluid in the introducing pipeline is sent back to the second return tube of the return pipeline and then returned to the fluid storage unit. a double diaphragm pump that includes a pneumatic mechanism mounted therein and provided with a first diaphragm, a second diaphragm, and an air-intake portion that brings air therein for driving the first and the second diaphragms (11, 12) to move and act through action paths, a first inlet and a first outlet that are disposed on one end of the pneumatic mechanism with the first diaphragm, a first fluid chamber formed between the first diaphragm, the first inlet and the first outlet, a second inlet and a second outlet that are disposed on another end of the pneumatic mechanism with the second diaphragm, a second fluid chamber formed between the second diaphragm, the second inlet and the second outlet, a plurality of check valves each of which disposed on the first inlet, the first outlet, the second inlet and the second outlet correspondingly; an introducing pipeline composed of a first guiding tube with one end thereof connected to the first inlet of the double diaphragm pump, and a second guiding tube with one end thereof connected to the first outlet of the double diaphragm pump; a return pipeline that includes a first return tube connected to the second inlet of the double diaphragm pump, and a second return tube connected to the second outlet of the double diaphragm pump; a fluid storage unit used for supplying fluid required and receiving fluid returned and connected to the first guiding tube and the second return tube; and a fluid supply portion that supplies fluid required to the machine and connected to the second guiding tube and the first return tube; wherein, in a first operation state, fluid in the fluid storage unit is passed through the introducing pipeline and the action path of the first diaphragm and delivered into the fluid supply portion to be used by the machine when the double diaphragm pump applies a pressure to the first diaphragm; fluid in the fluid supply portion is passed through the return pipeline and the action path of the second diaphragm, and returned to the fluid storage unit when the double diaphragm pump applies a pressure to the second diaphragm; characterized in that the fluid delivery system further comprises a drain-back tube arranged between the second guiding tube and the second return tube and provided with a control valve; wherein, when the control valve is closed, the fluid delivery system is in the first operation state; and wherein, when the control valve is open, the second guiding tube and the second return tube communicate with each other by the drain-back tube and the fluid delivery system is in a second operation state, in which the fluid in the introducing pipeline is sent back to the second return tube of the return pipeline and then returned to the fluid storage unit.