By considering the coupling effect between the healthy lower-limb and the passive prosthesis, this paper builds a heterogeneous dynamic model for gait analysis, where the motions of the healthy limb and the prosthesis are driven by the central pattern generator (CPG) and the hip joint swing, respectively. The foot-ground contact is modelled as the process of unilateral force reaction rather than the constraint to get a refined representation of the gait motion. The response of the heterogeneous model, solved by numerical calculation, is then analyzed by comparison with a real gait test. Preliminary results show that the heterogeneous model not only describes the amputee’s gait well but also reveals a new gait feature of period-doubling. Parameter analysis further indicates that the period-doubling gait will return to the single-period pattern by amplifying the vertical motion of the hip joint at the amputated side. This dynamic bifurcation, which mimics the process of hip swing adaption, provides new insight into the compensatory mechanism for lamely walking.