This study aimed to investigate the effects of HDPE and Cd on forms of carbon (C) and nitrogen (N) by measuring the changes in enzymatic activities and physicochemical properties of Spartina alterniflora soil samples from coastal regions of China. Over three incubation periods (14, 28, and 56 days), a soil incubation experiment was conducted to investigate the effects of HDPE (0, 0.5, 1, and 2 g per 100 g of soil) and Cd (0, 2, 4, and 8 mg kg⁻¹) on soil physicochemical properties. The results demonstrated that the sole presence of HDPE had a notable impact on enhancing the C-related physicochemical properties of the soil, particularly by elevating the concentration of Total Organic Carbon (TOC). The sole addition of Cd significantly suppressed enzymic activities in the soil, leading to a considerable reduction in the concentration of NH₄⁺-N and NO₃⁻-N. Under identical Cd treatment conditions, the introduction of 1 g HDPE led to an increase in the concentration of TOC, and the inhibitory effect of Cd on enzymic activities was decreased; thus, an elevated consumption of soluble organic carbon (DOC) was identified. However, upon adding 2 g of HDPE, while the TOC concentration continued to rise, the stimulatory effect on enzymic activities diminished. In conclusion, the addition of HDPE inhibits, to a certain extent, the influence of Cd on the carbon and nitrogen cycling in soil.