To investigate the causality between B cell count and psoriasis by Mendelian randomization (MR). Collected B cell count and psoriasis data from IEU Open GWAS Project. Employed inverse variance weighting (IVW), MR‐Egger, WM, weighted mode for analysis, ensuring result robustness. Assessed horizontal pleiotropy with MR‐Egger, detected outliers using MR‐PRESSO and examined instrumental variables heterogeneity with Cochran's Q‐test. The IVW method suggested an association between a genetically predicted memory B cell count and the risk of psoriasis vulgaris. IVW results also showed no causality between other exposure factors and the corresponding outcomes. Also, the global test of MR‐PRESSO analysis showed a significant association between a genetically predicted transitional absolute B cell count and the lower risk of psoriasis vulgaris. MR‐Egger regression showed that horizontal pleiotropy did not influence the analysis results. We found that memory B cell absolute counts are associated with a lower risk of psoriasis. These data further elucidate the role of memory B cells in psoriasis and provide new options for psoriasis treatment.