The electron momentum relaxation time is studied systematically in order to understand its effect during the excited nonlinear ionization process in fused silica with an irradiation of tightly focused femtosecond laser pulses. According to the analysis of a (3+1)-dimensional extended general nonlinear Schrödinger equation, the electron momentum relaxation time shows a huge effect on peak intensity, free electron density, and fluence distributions in the focal region of the incident pulse, meanwhile a value of 1.27 fs is thought to meet the present experimental result based on the theoretical model. Further research indicates that the change of electron momentum relaxation time can have significant difference among several nonlinear mechanisms, such as the laser-induced avalanche ionization, reverse bremsstrahlung, self-defocusing of plasma, etc. Results show that the electron momentum relaxation time plays an important role in the process of femtosecond laser pulses interaction with materials.