Abstract: This study investigated the antifungal properties and applications of lanthanum pyrithione (LaPT)/polypropylene (PP) composite films for the preservation of sliced bread. LaPT was synthesized via a coordination precipitation method, and a PP composite film containing 0.5% LaPT was prepared using melt blending technology. The antifungal mechanism against Penicillium chrysogenum (P. chrysogenum) and migration safety of the composite film were systematically analyzed through inhibition zone tests, minimum inhibitory concentration (MIC) assays, flow cytometry, and migration studies. The results demonstrated that LaPT exhibited remarkable inhibitory effects against P. chrysogenum, with an inhibition zone diameter of 27 mm and an MIC of only 2.5 μg/mL. Flow cytometry confirmed that LaPT exerts its antifungal activity by disrupting the integrity of fungal spore cell membranes. The 0.5% LaPT/PP composite film achieved an antifungal efficacy of 96.8% while maintaining the mechanical properties of the PP matrix. Migration studies revealed that the migration levels of LaPT in food simulants, including water, 4% acetic acid, 20% ethanol, and isooctane, were below the regulatory limits set by the European Union (EU No. 10/2011). Under storage conditions of 25 ℃ and 45% relative humidity, the 0.5% LaPT/PP packaging extended the time required for total microbial and fungal counts to exceed the acceptable limits by 4 and 8 days, respectively, compared to conventional PP packaging. Additionally, it effectively preserved the bread's moisture content at 29.1%, maintained a pH of 5.28, and reduced the extent of lipid oxidation to 0.17 g/100 g. Collectively, these findings demonstrate that the LaPT/PP antimicrobial packaging can extend the shelf life of sliced bread from 3 days to 9 days, providing a scientific basis for the development of novel antimicrobial packaging materials with significant potential in food preservation applications.