Surface electron escape probability (P) in quantum efficiency formula of GaAs photocathode commonly is taken for a constant independent of incident photon wavelength (λ) within photocathode working wave band. Quantum efficiency formula is applied in data fitting of spectral response curves of reflectionmode GaAs photocathode grown by molecular beam epitaxy, in which the epitaxial layer thickness is 1.6 μm, and doping concentration is 1×1019 cm-3. Data fitting results show that theoretical curves cannot tally with experimental curves completely, especially with declined spectral response curves of photocathode in activation chamber. This deviation is caused by the relation of P and λ, and P is not a constant independent of λ. Based on the data fitting analysis of spectral response curves, it is found that the relation of P and λ for reflection-mode photocathode approximately satisfies exponential function, and P connects with λ through surface potential barrier factor (k). Photocathode surface potential barrier factors after high- and low-temperature activation process are 3.53 and 1.36 respectively.