Biodegradable polymer (BDP) composites with functions of nutrient slow release and water retention have been widely concerned in the agricultural field, but the effects of soil heavy metals (HMs) on their degradation and slow release performances have not been reported. Tomato pot experiments were employed to study degradation and nutrient release characteristics of a slow-release nanocomposite prepared by potassium dihydrogen phosphate and urea-formaldehyde (namely PSRF), simple physical mixtures or semi-interpenetrating polymer network (semi-IPN) nanocomposites of PSRF and a superabsorbent polymer (SAP) in soil polluted by copper, zinc, cadmium and plumbum. Results show that the inhibitions of HMs on biodegradability and nutrient release of BDPs can be effectively reduced by combined with three-dimensional cross-linked hydrophilic SAPs, and their molecular segment scale mixing in semi-IPN composites possesses a more significant reduction effect than that of their simple mixtures. Meanwhile, the higher the number and activity of functional groups in introduced SAP, the more significant its adsorption and fixation to HMs, and the less the degradation inhibition caused by HMs. This strategy is suitable for the development of BDP-based composites, which can be applied to the simultaneous supplies of water and fertilizer in polluted soil.