Aiming at the characteristics of two-dimensional silicon grating microstructure in the near infrared light absorption rate is extremely low, a micro-nano composite structure is proposed to enhance the near infrared absorption of Ag nano-pit and silicon grating by using isolators. The absorption rate of the composite structure in the wavelength range from 0.78 μm to 2.5 μm is studied based on the finite-difference time-domain method. The influence of the grating structure and Ag nano-pits on the light absorption efficiency is analyzed. The simulation results show that when Ag nano pits with a diameter of 0.1 μm are embedded in the grating gap with a period of 0.2 μm and a duty cycle of 0.5 and on the surface of the grid column, the absorptivity of the composite microstructure is above 23% in the near infrared wide band, and the average absorptivity is up to 52.3%, theoretically. When Al₂O₃ dielectric layer is added on the grating surface, the absorption rate of wide band is above 41.3%, and the average absorption rate is increased to 65.1%. Finally, the absorption efficiency is improved in the wide wavelength range of near infrared, which provides a new method to enhance optical absorption in photodetector, solar cell, optical communication, radar stealth, biomedical and so on.