The invention discloses a rotary piston cylinder, which is a mechanism capable of positively / reversely converting fluid pressure energy and mechanical energy in a one-way continuous circular motion mode. The rotary piston cylinder mainly comprises a cylinder body (1), a rotor (2), a clapboard (3), a working medium inlet (4), a working medium outlet (5), a rotary piston (6) and a crankshaft connecting rod mechanism (7). The working principle of the rotary piston cylinder is the same as that of a vane type oscillating cylinder. The rotary piston cylinder is characterized in that: the working rotary piston (6) displaces on a plane which is parallel to the axis under the action of the crankshaft connecting rod mechanism (7), and can cross the clapboard (3) to apply work circularly and realize the one-way continuous circular motion mode. The rotary piston cylinder has the characteristics of high energy conversion efficiency, high bearing capacity, widely permitted working media, continuous and stable output, cascade connection capacity, load-carrying self-start, stepless speed change, high-speed response, fast reversing, overload protection, excellent clutch brake performance, and the like.