Abstract: As the first reactor test of the European Pressurized Water Reactor (EPR) Units, partial cooldown test would cause enormous thermal shocks in the primary and secondary coolant circuits, which are not allowed to happen more than 15 times during the service life of a nuclear power station. In order to lower the risks in the test, studies are furthered on the control logic, theory, principles and the feasibility of the test, and the risks are summarized in 5 aspects. Meanwhile, comprehensive analysis  is performed on partial cooldown test by simulations, and then the test method is optimized.The test succeeds at one time, and meets the nuclear safety requirements.