Quantum efficiency （QE） is one of the most important characteristics of photoemission performance of negative electron affinity （NEA） photocathodes. The quantum efficiency formulas for NEA photocathodes show the influences of photocathode parameters on photoemission performance and also provide theoretical guidance for optimum design of photocathodes. The photoemission process in bulk and surface of NEA photocathodes is firstly analyzed. Surface electron escape probability and attenuation length are respectively revised considering the effects of incident photon energy，surface band bending region and surface barrier on photoemission. The quantum efficiency formulas for NEA photocathodes are established by integration method. The theoretically estimated QE curve is consistent with the experimental one，which validates the practicability of the revised QE formula. This work can provide theoretical references for research of NEA photocathodes.