Abstract: The physical model of the 300# swimming pool research reactor(SPRR-300) based on the Monte Carlo code MCNP has been verified. Sophisticated modeling is conducted. An effective multiplication factor value of 1.002 29 is obtained, existing a relative error of 0.229% compared with the critical value. Meanwhile, a problem comes out that the interrupt and continue-run with parallel version of MCNP doesnt work. The problem is solved through trail and error process. A reasonable application of flux tally average over a cell and flux tally at a point is suggested, namely the former is prior to the latter to tally in big volume. Comparison between calculation results and experimental data shows that the thermal neutron flux has a deviation of 4.6% at a power level of 3 MW. That is to say, the calculated value and the experimental value agree well with each other, and the neutron flux result is dependable.