Abstract: Aiming at the radioactive gas compositions and monitoring requirements in the process of pressure relief and exhaust during the accident conditions of nuclear power plant, the theoretical design was made for the containment pressure relief and exhaust activity detector of the nuclear power plant. And the simulation computation was made for the performance of the monitor with the MCNP. The results show that under the conditions of choosing the reasonable measuring collimation aperture and shielding plan, the device can realize monitoring the total activity of containment pressure relief and exhaust. Through comparing the simulation computation results with the test data, it is shown that the results of simulation computation are credible, and provide an important theoretical support and reference for the development of the containment pressure relief and exhaust activity detector for the nuclear power plant.