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The large axial flow pump system ( LAPS ) used in coastal pump stations often needs to be equipped with overflow holes to improve the quality of the transition process. However, due to the unclear mechanism of adding overflow holes in the transition process of LAPS, there are many difficulties in design and application. Totally six kinds of overflow holes with different diameters were designed. Based on the secondary development of Flomaster software, the effects of overflow holes with different diameters on the synchronous start-up, asynchronous start-up, synchronous shut-up and asynchronous shut-up transition process of LAPS were studied by transient simulation method. The results showed that during the asynchronous start-up process, when the diameter of the overflow hole reached 2m, the maximum impact head was 1.67Hr, and the maximum impact power was 1.34Pr. Further increasing the diameter of the overflow hole had no significant gain in reducing the maximum impact head and power of the LAPS. In the process of asynchronous shut-up, the overflow hole can effectively delay the attenuation of LAPS flow and reduce the instantaneous head and power. However, when the diameter of the overflow hole reached 2m, the maximum impact head was 1.18Hr and the maximum impact power was 1.1Pr. Further increasing the diameter of the overflow hole had no obvious effect on reducing the maximum impact head and power during the asynchronous shut-up of LAPS. The larger the diameter of the overflow hole was, the better the effect on improving the quality of the transition process was, but when the diameter of the overflow hole was increased to a certain extent, the effect of continuing to increase the diameter of the overflow hole was not significantly improved.