The content of mineral elements is a crucial indicator for evaluating forage quality and breeding. In recent years, rapeseed has become a valuable high-yield forage source, playing a key role in alleviating the shortage of feed resources in China. This study aimed to reveal the accumulation patterns of major mineral nutrients in the core germplasm of Brassica napus and to support the breeding of mineral-rich rapeseed. We evaluated the phenotypic variation in the contents of K, Ca, Na, Mg, Fe, Zn, and Cu in 280 core germplasms of B. napus using ICP-OES after hydroponic treatment. Through GWAS, we identified molecular markers significantly associated with the content of these minerals and predicted related candidate genes. The results showed significant variation in mineral content among different germplasms, and principal component analysis transformed the seven mineral indicators into three new indicators, with a cumulative contribution rate of 68.479%. Significant positive and negative correlations were found between different minerals; some germplasms exhibited extreme values for multiple minerals. A total of 134 SNPs related to mineral content were identified, and 14 candidate genes were predicted. This study provides important insights for the breeding and creation of new germplasm rich in mineral nutrients.