The rapid pace of urbanization has increased the development and land reuse value of China’s closed industrial solid waste landfill (CISWL) sites. In-situ development of land without waste excavation has both economic and environmental benefits. Therefore, the present study systematically evaluates the on-site risks of a CISWL site under in-situ development conditions by sampling analysis and process model simulation to predict its long-term evolution. The results demonstrated that 93% of the waste leaching concentration was harmful and 66% of the waste was not suitable for soil directly used for urbanized land after nearly 20 years of leaching and degradation. The groundwater quality had changed from “no possibility of exceeding the standard (short term)” to “only a few harmful substances (Pb) have a very small possibility of exceeding the standard (medium term),” to “both Pb and total cyanide (T-CN) have a high probability of exceeding the standard (later stage).” At the same time, the health risks of the site use process have gradually increased over time. The carcinogenic risk posed by As and the non-carcinogenic risk from T-CN exceeded the acceptable level by 139–267 times and 4.9–17.2 times, respectively. The above results suggest that risk assessment and management strategies of CISWL site development and reuse should focus on long-term risks. For sites with unacceptable long-term risks, acceptable long-term risks can be lowered appropriately by reducing the concentration of toxic substances in leached waste. The present study proposes a calculation framework and method for the corresponding leaching concentration limits. •Harmful pollutants degrade slowly in closed landfill environment.•Long-term health risks exceed 4.9–267 times the acceptable level for this site.•Long-term risks should be considered when reusing industrial solid waste sites.