Breaking Lorentz reciprocity is one necessary condition of optical isolator design. Unidirectional wavelength-mode conversion will be realized in a time-dependent system through a short operating range. Based on plasma dispersion effect, generate space-asymmetric periodical time-space modulation on silicon waveguide, and non-reciprocal propagation is realized in the waveguide. The designed unidirectional wavelength-mode conversion waveguide demonstrated that in the forward direction, input 1.55 mu m fundamental mode light signal and then output 1.5492 mu m is of 1st-order mode, while in the backward direction, input 1.5492 mu m is of 1st-order mode light signal and then output 1.5484 mu m is of fundamental mode. Based on this non-reciprocal structure, mode conversion waveguide and two-ring resonance filters were designed then, to accomplish on-chip optical isolation. The scale of the designed isolator is 160 mu m x 60 mu m, and the isolation is 21 dB, revealing perfect application potential.