The utility model discloses a tail gas straight-flow engine which comprises at least one air cylinder which is provided with an exhaust port in one side face, a cylinder body which is detachably connected to the air cylinder and is in airflow communication with the air cylinder, and an air inlet valve which is detachably connected to the cylinder body and is located on one side face of the cylinder body so that an airflow can flow into the air cylinder and burned in the air cylinder from the air inlet valve by virtue of the cylinder body, wherein the burned tail gas can be discharged from the exhaust port; and an included angle formed between the direction that the airflow flows into the air inlet valve and the direction that the tail gas is discharged out of the exhaust port is 0-90 degrees. The straight-flow engine disclosed by the utility model can solve or partially solve the technical defect that the flying operation of an unmanned aerial vehicle is affected in the case of insufficient gas combustion of a conventional engine.