There is no reference reactor for CARR and no zero power experiment carried out, the nuclear commissioning of CARR will be run based totally on the theoretical analysis, as the first case in the China's domestic research reactor start-up. Because of the complicated structure and the large variations of the load of the core, the simulation calculation of the experiment can not be carried out with most neutronics codes based on the diffusion theory. In this paper, the MCNP code is used to run the calculation because of its real geometry function. The calculation results provide some important reference for the CARR start-up. The results of the experiment show that the calculation results are accurate and reliable and the application of the code is successful.