Photocatalytically degradable plastics (PDP) are promising in mitigating plastic pollution due to their high degradability. Nevertheless, the unclear impact of environmental conditions on properties of debris for PDP and the knowledge gap in the mechanism from the perspective of photogenerated reactive species’ function hinder PDP's application. Here, low-density polyethylene (LDPE) containing TiO₂ (LDPE-TiO₂) was selected as representative PDP. The photocatalytic degradation performance and mechanism of LDPE-TiO₂ in water without NaCl, water with NaCl, and air were studied. Compared with water, the air resulted in a higher carbonyl index and higher molecular weight of debris from LDPE-TiO₂ whether the irradiation time was the same (120 h) or the mass loss of LDPE-TiO₂ was the same (68 ± 2%). Chloride ions in water conditions resulted in CCl bond formation in the debris of LDPE-TiO₂ suggesting generation of organochlorine substance. In the air the photogenerated electron (e⁻) contributed to LDPE-TiO₂ film degradation, while in the water e⁻ mainly underwent recombination with the photogenerated hole. In water with Cl⁻, HO‧ attacked LDPE via reaction with Cl⁻ forming organochlorine substance. The effects of environmental conditions on the properties of debris and the functions of photogenerated reactive species for PDP degradation are reported for the first time in this study. The findings provide scientific support for the development of PDP technology.