The DuPont financial analysis system, with net interest margin as the core, uses debt - paying and operational efficiency indicators to analyze listed companies' profitability. But it's mainly for static analysis, lacking in dynamic analysis, which the chain substitution method can remedy. This paper centers on the DuPont system, analyzes Toly Bread's 2021-2023 annual reports, calculates relevant financial indicators, and improves them with the chain substitution method. Financially, it spots Toly Bread's operational issues, offers improvement suggestions, aiming to aid Toly Bread and related food firms' further development.