y using the hyperbolic grading function and taking account of the potential varies near the surface of photocathodes caused by an applied bias, the Conduction band structures at the heterojunction interface for a field-assisted InP/InGaAsP/InP Semiconductor photocathode are analysed and calculated in this paper. The profile curves of conduction band structure at the heterojunction interface with the different parameters of materials have been obtained. In order to achieve the perfect transmission efficiency at heterojunction, the calculation resultes have shown the conditions for the thickness and the doping concentrotion of the emission layer, the doping concentrotion of the absorption layer, as well as the grading width at the heterojunction interface and the applied bias. The results are helpful to designing field-assisted semiconductor photocachode and choosing material parameters.