Abstract: The hydraulic resistance of the moving body within hydraulic suspended passive shutdown subassembly (PSS) for sodium cooled fast reactor (SFR) in the process of falling and its calculation method were analyzed. Based on the internal flow path of PSS, the flow rate distribution and hydraulic resistance distribution during PSS moving body suspension and falling process were analyzed and modeled. The motion process of the moving body was calculated by using quasi-steady state combined with transient correction method. Based on the above calculation method, the time course analysis of the falling motion of the moving body during the unprotected loss of flow accident was completed. The result shows that during the falling of the moving body, the velocity and accelerated velocity rise, and the hydraulic resistance drops. The moving body which drops for 0.588 m needs 6.68 s. The calculation result of moving body falling time meets the preliminary design requirement and can be used as a reference for PSS design.