According to the ratios of the peak-values of photocurrents arising separately during the single Cs activation and during the Cs-O activation for negative electron affinity(NEA) GaAs photocathode, and the theoretical energy distributions of the photoelectrons passing separately through the single and the double potential barriers, a new method of evaluating the surface potential barrier parameters of NEA GaAs photocathode is presented. The results obtained by this method accord well with the double-dipole model theory and are in agreement with the results by fitting the experimental electrons energy distribution curve. The method is simple and efficave, which enriches the approaches to the evaluation of activation effect and surface characteristic of NEA GaAs photocathodes without increasing other test means.