Abstract: Based on the idea that the nuclear fission reaction occurring and delayed fission neutron emitting can be separated in time dimension, the delayed fission neutron time spectrum in the range of 0-100 s in uranium contained formation was calculated by MCNP. Firstly, the ²³⁵U and ²³⁸U nuclear fission reaction space distributions in sphere geometry were simulated. Then delayed fission neutron’s emission position and direction, originated fission nuclide, delayed fission neutron group and emission time, and initial energy were sequentially sampled with the assistance of SOURCE subroutine in MCNP. The result shows that the time feature of the calculated delayed fission neutron time spectrum is in accordance with the group parameters of delayed fission neutron of uranium nuclides. The relationship between accumulative delayed fission neutron count and formation uranium concentration was linearly fitted. The adjusted R square of fitting result is greater than 0.998 5, which verifies the credibility of uranium concentration linear model.