Modeling and simulation of multi-alkali photocathode have been studied. The stucture of photocathode is employed with layered model: Na2KSb+K2CsSb+Sb·Cs dipole layer. Effects of the thickness and doping concentration of each layer on the energy band and spectral response character have been discussed. The result shows that when the N-doping concentration in the K2CsSb and Sb·Cs layers is heavier, it can effectively lower the electron affinity, which will be helpful to the transport and escape of the photoelectron. The doping concentration of Na2KSb is not that the higher the better, which comes from the effect of doping concentration on build-in electric field intensity and range. The diffuse distance of electron will get further when under higher build-in electric field intensity, which can make electron have higher probability to reach the surface of photocathode. The highest sensitivity is got when doping concentration is about 1016 cm-3. Influence of thickness on sensitivity is analysed, for a certain wavelength incident light, there exists the optimum thickness. Under the impact of the build-in electric field, different doping concentrations have the corresponding thickness. With higher build-in electric field, the optimum thickness enhanced, meanwhile. For 700 nm incident light, when doping concentration is 1017 cm-3 or 1016 cm-3, the optimum thickness is 80 nm or 200 nm, respectively.