This paper focuses on the topic of trajectory generation of lower limb exoskeleton by a proposed central pattern generator (CPG) algorithm. By combining oscillators, i.e. dynamical system that exhibit limit cycle behavior, a CPG model for lower limb exoskeleton is proposed. Such a model can generate kinds of trajectory and achieve their smooth transition. Moreover, in order to realize the synchronization between the user and exoskeleton, an additional module is introduced which acts as a waveform learner of arbitrary periodic signal. Numerical simulations are carried out, we demonstrate that this module is robust and has good performance in generating trajectory.