As a new type of nuclear power system, more attention has been paid to the small modular reactor in the development of nuclear power after the Fukushima accident. This paper focuses on the core study of a 5-MW(thermal) water-cooled super small modular reactor (WSMR) based on the sensitivity analysis of the key design parameters. The key design parameters include the fuel enrichment of 235U, the moderation performance, the separation work, and the layout scheme of the control rods. The impact analysis of the key design parameters on the steady-state performance and on the burnup performance is provided. From the impact analysis, the method of how to extend the operation life of the WSMR reactor is provided.