At present, all-silicon photoelectric biosensors based on silicon-based light sources and compatible with complementary metal oxide semiconductor (CMOS) technology have low luminous efficiency of the light source, resulting in low detection sensitivity, and cannot be applied on a large scale. In response to this problem, the polysilicon PN junction cascade light source and a new all-silicon photoelectric biosensor are researched, including the luminescence mechanism of the PN junction cascade polysilicon light source, waveguide transmission mechanism and waveguide biological detection technology. The developed polysilicon PN junction cascade light source has a luminous efficiency of 4.3×10-6, which can be efficiently coupled with a waveguide. Simulation results show that when the refractive index of the medium in the detection area of the waveguide changes between 1.33 and 1.73, the increase in the refractive index of the medium makes the light energy less return to the inner core of the waveguide, so the transmitted light intensity in the inner core of the waveguide decreases with the increasing in the refractive index of the medium. And medium refractive index sensing is realized through the changing of light intensity.