Breast cancer is a malignant disease that seriously threatens women’s health. Studying the mechanism of cancer occurrence and development is an urgent problem to be solved. In this paper, the eigen microstate method was used to study conversion of normal breast cells into breast cancer cells and the reason. The main conclusions are as follows: the microstates of normal breast cell and breast cancer cell are different. There is a state conversion when a normal breast cell transforms into a breast cancer cell. The main reason for this state conversion is the combined effect of tumor suppressor genes and oncogenes. By analyzing the function of key genes, it was found that these genes do play an important role in the development of breast cancer. The findings contribute to understanding the mechanism by which breast cancer occurs and progresses, and key genes can serve as potential biomarkers or target genes for breast cancer treatment.