Abstract: To evaluate the detection cases and predict experiment results rapidly, a fast estimation model of neutron cell flux was established. Theoretically, the neutron cell flux in an enclosed space is inversely proportional to the square of the characteristic length, and the magnitude of neutron cell flux can reflect the intensity of scattering neutron flux. The neutron cell flux in enclosed experimental hall was calculated by Monte Carlo method, and the scattering neutron flux at different distances from neutron source was obtained. The relationship between the neutron cell flux and the hall size is obtained by fitting the simulation results. The relationship between the scattering neutron flux at different distances from neutron source x and the hall size L is also fitted, and the relative deviation between the fitting results and Monte Carlo simulation results is within 10%. The results show that the neutron cell flux in spherical structure is inversely proportional to the 1.905 power of the sphere radius; the neutron cell flux in strip experimental hall is inversely proportional to the 1.948 power of cross section width of experimental hall, also inversely proportional to the 0.775 power of ratio of length to width of experimental hall. The average track length of each single source neutron in spherical structure is 5 times the sphere radius, while in the enclosed experimental hall, the average track length of each source neutron is 2~3 times the characteristic size of experimental hall.