A method for operating the auto/single functions selecting device of a grease gun, comprising: step (a): having a primary path, an air release path and a recess defined in a handle of a main body of a grease gun, the primary path communicating with a chamber; step (b): installing a valve unit in the recess and the valve unit having a return path, the valve unit controlling air being introduced into or released from the chamber; step (c): having a third air path communicating between the primary path and the recess, and step (d): a switch unit installed in the third air path, a single function being activated when the switch unit closes the third air path, an auto function being activated when the switch unit opens the third air path; characterized by the fact the valve unit is located in the recess and comprises a first valve, a first passage, a second valve, a second spring and a rod so as to form the return path, the rod partially protrudes from the recess and is connected with a trigger, when the primary path is opened, a right push force P4 is applied to the second valve and the second spring applies a left push force P3 to the second valve, when the return path is opened, a left push force P1 is applied to the second valve, when the switch unit opens the third air path, a left push force P2 is applied to the first valve, when the switch unit is switched to the auto function and the primary path is connected with a pressurized air source, the sum of the left push force P1 and the left push force P3 is larger than the right push force P4, so that the second valve seals the primary path while the air release path is opened, when the rod is pushed to seal the return path, the left push force P1 is disappeared, the primary path and the third air path apply a left push force P2 to the first valve, the return path is opened to activate the auto function. step (a): having a primary path, an air release path and a recess defined in a handle of a main body of a grease gun, the primary path communicating with a chamber; step (b): installing a valve unit in the recess and the valve unit having a return path, the valve unit controlling air being introduced into or released from the chamber; step (c): having a third air path communicating between the primary path and the recess, and step (d): a switch unit installed in the third air path, a single function being activated when the switch unit closes the third air path, an auto function being activated when the switch unit opens the third air path; characterized by the fact the valve unit is located in the recess and comprises a first valve, a first passage, a second valve, a second spring and a rod so as to form the return path, the rod partially protrudes from the recess and is connected with a trigger, when the primary path is opened, a right push force P4 is applied to the second valve and the second spring applies a left push force P3 to the second valve, when the return path is opened, a left push force P1 is applied to the second valve, when the switch unit opens the third air path, a left push force P2 is applied to the first valve, when the switch unit is switched to the auto function and the primary path is connected with a pressurized air source, the sum of the left push force P1 and the left push force P3 is larger than the right push force P4, so that the second valve seals the primary path while the air release path is opened, when the rod is pushed to seal the return path, the left push force P1 is disappeared, the primary path and the third air path apply a left push force P2 to the first valve, the return path is opened to activate the auto function.