Abstract: Radioactive substances will be released into the primary coolant when fuel element of marine reactor is damaged occasionally. If these radioactive substances cannot be effectively removed, the leakage of radioactive substances in the pipe break accident may cause the crew to suffer radioactive radiation damage. The power plants usually use a pressurizer spray to remove the inert gas in the system. Since there are differences in the structure and operation characteristics between the ship reactor and the power plant reactor, it is necessary to study the removal effect of radioactivity by spray degassing of pressurizer. The research can provide guidance and assistance for the radioactive safety management strategy under the condition of small package shell damage. MELCOR, as a common severe accident system analysis program for PWR, can simulate the thermal hydraulic process under accident and the migration and diffusion process of radioactive substances accompanying the accident process. In this paper, the MELCOR program was used to model small ship reactor about the spray degassing process of the pressurizer and shutdown process. Then the whole physical process was simulated which comprised of power operation, power reduction degassing and elimination of the pressurizer’s chamber after reactor shutdown. The rationality of the simulated physical process was verified by the simulation of the changing trend of the key operating parameters of the reactor. Combined with the established simulation model of degassing and shutdown, the influence of pressurizer spray degassing and shutdown process on the quantity of radioactive substances was calculated and analyzed, furthermore the influence on the radioactive removal through exhaust gases at pressurizer’s high point and degassing of spray was estimated. The results show that the removal amount of inert gas in spray degassing process and shutdown process accounts for 11.6% and 27.4% of the total inert gas respectively, so the total removal amount of inert gas is 39.0%; Without spray degassing before shutdown, the radioactive removal amount during shutdown is 30.3% of the total amount of inert gas; Degassing before shutdown is 8.7% more than the removal of inert gas without degassing before shutdown. In summary, the reduction of the radioactivity in the pressurizer by the pressurizer spray degassing is limited. Taking into account the adverse effects of degassing on the operation safety during operation, it is recommended that only the high point gas after reactor shutdown be discharged to remove the radioactivity.