Abstract: Brassica napus is the main planting type of rapeseed in China, a series of complex metabolic and regulatory processes were involved in its flower bud development, especially the early development process before microspore maturation, which has a significant impact on rapeseed fertility. Based on phenotypic and histological observations, the flower bud size between 1 mm and 3 mm of Brassica napus was found corresponding to the 5^th-9^th stages of angiosperm stamen development. Using the high-throughput UPLC-MS/MS mass spectrometry sequencing technology, proteomics analysis was conducted on these flower buds, and a total of 13444 proteins were identified, including 613 differentially expressed proteins (DEPs). Functional analysis of the DEPs revealed that there was a significant upregulation of cell wall building, lipid metabolism, and cell transport related proteins, which would be beneficial for cell building and pollen development during early stage of the flower bud development. The expression of nucleic acid related proteins changes greatly, which is consistent with the rapid cell division and growth during this period. However, no significant changes were found in the expression of proteins related to the ABCDE development model, these proteins may have other functions besides inducing flower bud differentiation. This study systematically studied the differences in tissue morphology and protein expression during the early stages of bud development in Brassica napus, it paves the way for further research on the metabolism and regulation of flower bud development.