Built-in electronic field has important effect to improve the quantum efficiency of the photocathode. In order to get a stable electronic field,the exponential-doping method is applied in the active layer. Exponential doping actually is a special gradient doping,according to this consideration,a new quantum efficiency formula including uniform doping and exponential doping is brought forward. Four doping samples with concentration from 1019 to 1018 cm-3 are produced in the experiment,meanwhile their spectral response curves and quantum yield curves are detected. Through the data fitting analysis of the quantum yield curve with three methods which are uniformly-doping quantum yield formula fitting,exponential-doping quantum yield formula fitting and the new-built graded-doping quantum yield formula,the fitting precision of the new-built quantum yield formula is proved best.