The invention discloses an automatic amplitude regulation leg rehabilitation training device and a control method. The device comprises a fixing support frame, a leg support and a pedal device, a liftable armrest frame and a seat fixing frame are installed at the front end and the rear end of the fixing support frame in a high-level manner, a pedal wheel axle is installed on the upper front side of the fixing support frame, a leg swing amplitude regulation device is arranged on the front surface of the seat fixing frame, a leg regulation device is installed on a swing frame in the leg swing amplitude regulation device, one end of the leg support is connected with the pedal device, the other end of the leg support is connected with the leg regulation device, and the pedal device is connected with the pedal wheel axle. According to the method, rotating angles are measured by an angle sensor arranged at a rotating position, and a sensing control system controls each magnetorheological damper, a magnetorheological clutch, a current contactor and a motor according to measuring signals.