A new way to calculate the quantum yield of negative electron affinity (NEA) photocathode is introduced. On the base of “three-step emission model”, the quantum yield of NEA photocathode is obtained by integral. The quantum yield formulas of reflection-mode and transmission-mode NEA photocathode are given. The formula of reflection-mode photocathode is the same with that derived from the diffusion equation. And, according to the formula of transmission-model NEA photocathode, thoretical quantum yield curve is drawn by computer, which is coincident with the curve drawn according to traditional formular and also identical with experimental curve.