【Objective】Biodegradable plastics are widely used due to their advantages, including clean raw materials and good degradability. However, there is a lack of quantitative evaluation on the dynamic variation laws of the physicochemical properties of biodegradable plastics during the photodegradation process, the formation mechanism of secondary pollutants, and their environmental fate.【Method】This study investigates the degradation dynamics and the release of harmful substances such as microplastics and additives during a 0-30 day photodegradation process of three typical biodegradable plastic films (starch-based, cellulose-based, and polylactic acid) and one conventional petroleum-based polyethylene (PE) film through simulated sunlight accelerated degradation experiments.【Result】Scanning electron microscopy (SEM) observations showed that the density of photogenerated cracks on the surface of biodegradable plastic films increased by 2-3 times compared to the initial state at 30 days. Fourier transform infrared spectroscopy (FTIR) revealed that the carbonyl index (CI) increased by 50-80% (with a significant enhancement in peak intensity at 1078 cm^-¹), and X-ray diffraction (XRD) analysis indicated a 20-35% decrease in crystallinity. The three materials had similar structures, while conventional petroleum-based plastics, dominated by C-C bonds, showed relatively weak environmental degradation capacity. Quantitative analysis found that among the four materials, the starch-based biodegradable film released the highest number of microplastics, exceeding 7×10⁷ pieces. Gas chromatography-mass spectrometry (GC-MS) detection showed the migration of phthalate esters (PAEs) additives in the biodegradable films, with the release of both di (2-ethylhexyl) phthalate (DEHP) and dibutyl phthalate (DBP). Although the release did not exceed the quality standards specified for 6 Phthalate esters in Water Quality-Liquid Chromatography-Triple Quadrupole Mass Spectrometry(HJ 1242-2022), it still poses potential environmental risks.【Conclusion】In summary, although biodegradable films have the characteristic of rapid apparent decomposition, secondary pollutants such as microplastics, dissolved organic matter (DOM), and phthalate esters generated during their photodegradation process may form a composite pollution chain, posing potential threats to the soil-water ecosystem. Thus, it is recommended to strengthen control over the degradation and life-cycle risk assessment in material design and standard formulation.