In order to explore the relationship among environmental regulation, the transformation of resource-based cities, and spatial spillover of resources, a semi-parametric threshold spatial Durbin model (STSDM) was constructed based on data from 113 resource-based prefecture-level cities from 2011 to 2021, starting from the perspective of spatial spillover. The study investigated the nonlinear relationship of environmental regulation with the transformation performance of resource-based cities and its asymmetric spatial spillover effects. The results show: ① Under the linear assumption, environmental regulation is not conducive to improving the transformation performance of cities. However, under the nonlinear assumption, environmental regulation is beneficial to improving the transformation performance of resource-based cities in the early stage. It exhibits a non-linear long-term characteristic with a "diminishing marginal effect", and the partial derivative plot shows a weak "U-shaped" feature; ② There is asymmetry in the spatial spillover effects on the transformation performance of resource-based cities; ③ The higher the stage of environmental regulation in resource-based cities, the fewer spatial spillover dividends they can obtain for the transformation of urban performance. Based on this, policy suggestions are put forward, such as tailoring environmental regulation policies to local conditions and establishing a long-term environmental protection mechanism.