Equations for surface photovoltage spectroscopy were deducted,by solving the ondimensional diffusion equation for equilibrium minority carriers of transmissionmode GaAs phtotocathode.Through measuring the surface photovoltage curves for GaAs photocathodes with the active layer thichness of 1.6 and 2.0μm,doping concentration of 1×1019 cm－3,experiments and fitting curves fit very well.By leading the formulas for integral sensitivity of surface photovoltage spectroscopy,the inflection of active layer thickness for integral sensitivity was analyzed under certain body parameters through emulations.It was found that GaAs photocathodes have a optimal active layer thickness and the backinterface recombination velocity inflects more on optimal thickness than electron diffusion length.Furthermore,GaAlAs window layer could help to well reduce the backinterface recombination velocity for active layer thickness.