The invention discloses a piston type air motor, comprising a cylinder body, a piston, a cylinder, a bent axle, a connecting rod, a camshaft, an on-off device, wherein the on-off device is used for controlling an air valve; a cylinder cover is arranged on the cylinder body; inwardly-opened pressure gas inlet valve/outlet valve are arranged on the cylinder cover; an inlet cam and an outlet cam arearranged on the camshaft; the on and off of the inlet valve and the outlet valve is controlled by a rocker arm which is driven by the camshaft, wherein the camshaft is driven by the rotating bent axle via a timing chain or belt, and then high-pressure gas can be controlled to enter the cylinder to promote the reciprocating motion of the piston and to discharge low-pressure gas which completes work; the piston outputs power by driving the bent axle to rotate through the connecting rod. The piston type air motor is characterized in that a suction valve is also arranged on the cylinder cover, a suction cam is also arranged on the camshaft, the on and off of the suction valve is controlled by the rocker arm so as to control the timing for the cylinder to suck and close the air outside, and the ratio of the diameters of the cylinder to the piston stroke is 1:2-15, most preferably 1:10.