The invention belongs to the technical field of armed robots and particularly relates to a simple three-DOF (degree of freedom) small-inertia bionic leg driven by hydraulic motors and applied to armed robot. The bionic leg comprises a hip joint X-direction hydraulic motor, a buttock part, a knee joint hydraulic motor, a hip joint Y-direction hydraulic motor, a thigh, a shank and a knee joint drive rod. The simple three-DOF small-inertia bionic leg has the following benefits: the structure is simple, two of three joints are directly driven by the hydraulic motors, and flexible motion of the leg is realized; the rotation inertia is small, and the three hydraulic motors are concentrated around the buttock part; a joint angle can be measured by a coding disc on each hydraulic motor; the structure of the thigh and the shank is simple, parts can be reduced, and the strength is improved; the joint torque is not changed with change of the joint angle, and output torque is stable; the power density of the hydraulic motors is high; the rotation angle of each joint can be set flexibly.