An auto-focusing projection system comprising: a projection module comprising a projection light source and a projection objective, light emitted from the projection light source traveling through a light splitting element in the projection objective and a first lens group in turn, and then emitting out; an infrared monitoring module comprising a monitoring objective and a sensing chip, an infrared source provided on the projection module, an IR light spot emitted from the infrared source falling within the monitoring scope of the infrared monitoring module, an IR light of the IR light spot reflected back travelling through the first lens group, the light splitting element in turn, and then into the infrared monitoring module; a focusing device for driving the first lens group moving forward and backward, the focusing device comprising a focusing control module and a focusing motor, the focusing control module receiving a real-time IR monitoring signal from the infrared monitoring module, and emitting a control signal to the focusing motor, the focusing motor being connected to the first lens group driving the first lens group to move forward and backward; the optical axis of the infrared monitoring module being coaxial with that of the projection objective after passing, from the sensing chip, through the light splitting element, characterized in that the optical characteristics of the projection objective and the monitoring objective ensure that the projection objective focus clearly when the first lens group is driven to move forward and backward by the focusing device to make the IR monitoring picture received by the sensing chip to be focused clearly and to make the projected picture to be focused clearly simultaneously; wherein the infrared light source comprises a collimated laser source and an emerging angle of less than 2 degrees, wherein the light spot keeps clear at any distance within the scope of a working distance of the projection system such that it is eligible for being a reference during focusing. a projection module comprising a projection light source and a projection objective, light emitted from the projection light source traveling through a light splitting element in the projection objective and a first lens group in turn, and then emitting out; an infrared monitoring module comprising a monitoring objective and a sensing chip, an infrared source provided on the projection module, an IR light spot emitted from the infrared source falling within the monitoring scope of the infrared monitoring module, an IR light of the IR light spot reflected back travelling through the first lens group, the light splitting element in turn, and then into the infrared monitoring module; a focusing device for driving the first lens group moving forward and backward, the focusing device comprising a focusing control module and a focusing motor, the focusing control module receiving a real-time IR monitoring signal from the infrared monitoring module, and emitting a control signal to the focusing motor, the focusing motor being connected to the first lens group driving the first lens group to move forward and backward; the optical axis of the infrared monitoring module being coaxial with that of the projection objective after passing, from the sensing chip, through the light splitting element, characterized in that the optical characteristics of the projection objective and the monitoring objective ensure that the projection objective focus clearly when the first lens group is driven to move forward and backward by the focusing device to make the IR monitoring picture received by the sensing chip to be focused clearly and to make the projected picture to be focused clearly simultaneously; wherein the infrared light source comprises a collimated laser source and an emerging angle of less than 2 degrees, wherein the light spot keeps clear at any distance within the scope of a working distance of the projection system such that it is eligible for being a reference during focusing.