In order to study the propagation characteristics and energy density distribution of femtosecond laser beam quality factor M2 based on Gaussian beam, on the basis of theoretical study of femtosecond laser beam quality factor, the corresponding data were calculated. The measurement method of annular spot width formed by femtosecond laser pulse irradiating the screen surface was given. An experimental platform consisting of femtosecond laser, lens and dielectric glass screen was established. The method was compared with the knife-edge method and the CCD method.The range of laser beam quality factor at different positions of femtosecond laser beam waist was confirmed by knife-edge method and CCD method. The results show that the measured values of beam quality factor M2 in x and y directions are 2.04 and 1.24, respectively. The experimental results are basically consistent with the theoretical analysis. Compared with knife-edge method and CCD method, the structure is simple. The data obtained are reliable and easy to carry out. It has certain reference value for precise measurement.