An in-prosthesis linear drive system includes a holder frame, a power drive mounted on the holder frame, a screw nut mounted on the holder frame and coupled to the power drive and rotatable on the axis thereof by the power drive, a screw rod limiter including a center sliding guide rail mounted on the holder frame and a center sliding block slidable along the center sliding guide rail, and a screw rod threaded into the screw nut and connected with the center sliding block of the screw rod limiter and movable along the axial direction of the screw nut upon rotation of the screw nut. Thus, using the screw nut to drive the screw rod achieves the effect of minimizing the occupation of the prosthesis internal space.