The effect of Co doping concentration on the optical properties of graphite-like C3N4(g-C3N4) were investigated based on experiment and simulation. The X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra show the prepared samples are g-C3N4 and Co doped g-C3N4. The UV-Vis diffuse reflectance spectroscopy (DRS) shows heavily Co doped g-C3N4 (0.15 wt%) exhibits better absorption characteristic in the wavelength range 450-800 nm. Using first principles based on density functional theory, the undoped and Co doped C3N4 were built and studied. Compared with others, the heavily Co doped g-C3N4 (C3Co0.375N3.625) exhibits excellent optical properties such as absorption and photoconductivity.