Abstract: PWR fuel assembly holddown spring systems generally consists of three to five spring leafs. To analyze the stiffness of the holddown spring system, commercial FEM code is usually used for the evaluation currently. But the optimization capability of the main current commercial FEM code is weak, during the research and development progress, it usually cannot screen a better performance design product. This paper established the holddown spring stiffness analysis model from the perspective of theoretical analysis. Using the established theoretical analysis model, the paper gives three main factors impacting the holddown spring stiffness: leaf thickness, leaf width and leaf height, and also give the analysis on how these factors affecting the holddown spring stiffness.