As the operating time of domestic pressurized water reactor (PWR) nuclear power units increases, the collective dose of the primary circuit in China’s nuclear power units has shown an upward trend, with some units’ collective dose becoming a weak link in WANO indicators. Research and practical experience have shown that zinc injection technology in the primary circuit can make the oxide film formed on the surface of primary circuit equipment more stable. The addition of zinc effectively suppresses the release of elements such as nickel and cobalt into the primary circuit coolant, and also reduces the deposition of cobalt-58 and cobalt-60 in the oxide film on the surface of primary circuit structural materials, thereby reducing the radiation dose during shutdown. This paper outlines the on-site implementation process of primary circuit zinc injection technology during the hot functional test of Hualong Nuclear Power Plant Unit, and introduces the characterization analysis of on-site hanging samples and the effects of zinc injection. The results indicate that during the hot functional test, zinc injection promoted the formation of a denser oxide film on the surface of primary circuit structural materials.