The invention discloses an auxiliary water supply system for a nuclear power station steam generator. The auxiliary water supply system comprises a water storage tank and a centrifugal pump which are connected through multiple pipelines to form a circulation loop, a pipeline at an upstream inlet of the centrifugal pump is provided with a siphon connected with a medicine barrel, and a pipeline at a downstream outlet of the centrifugal pump is provided with a purifying device. According to the auxiliary water supply system, the inlet of the centrifugal pump is provided with the siphon connected with the medicine barrel, through siphoning before pumping, the medicine can be filled into the pipeline, the water storage tank is cleaned by utilizing power cycle of the centrifugal pump, online medicine filling is realized, risk operation such as hoisting and high altitude dosing can be avoided, and advantages of safety, convenience and rapidness are realized. The downstream outlet of the centrifugal pump is provided with the purifying device equipped with a detachable and washable filter core, and purifying demands of the water storage tank can be satisfied.