In the Yellow River Delta in China, mulch residue from croplands may pollute surrounding soil and near-shore waters. However, there is a lack of knowledge regarding the status and environmental pollution effects of mulch residue in this region. This study examined mulched croplands within 5 km of the main entering-sea rivers of this region and obtained 170 samples and 34 questionnaires. Using multiple analytical methods, we identified the key drivers of mulch residue characteristics, the effects of different types of mulch residues on soil, and potential offshore pollution zones. Mulch residue pollution in the study area was found to be heavily present (mean value of 51 kg/ha), posing a high risk of offshore pollution along the southeast and northwest coastlines. Large mulched fields with low residue levels and small debris showed a scale effect in mitigating mulch residue pollution. PBAT mulch usage was prone to the transport of small-sized residues with PE and other hard-to-degrade substances, indicating an increased offshore pollution risk. Large agglomerates in the soil, formed after using PE mulch, had poor water stability and low soil organic carbon content. Mulch residues significantly reduced the abundance of the soil fungi Ascomycota and Basidiomycota. In order to alleviate the negative effects of residual mulch, it is necessary to carry out large-scale mulching and use degradable mulches and easily recyclable thick PE mulches in the future.