Abstract: This paper describes the ANDRITZ coolant pump shaft seal system of Fangjiashan nuclear power plant by using the operation data obtained in the functional experiments. The paper also analyzes the operational risk of seal injection water, the effects on the running main pump when the main valve status is changed, and, in addition, the effects on the starting or running main pump when each level of main shaft seal system’s parameters is changed. The results show that the seal injection water has certain operational risks, while the main system pressure has little effect on the high or low pressure leakage of the seal injection water. The main system pressure should be higher than 2.75 MPa in order to make it safe when the main pump is started.