Motorcycles are a highly unstable element that impacts road traffic safety due to insufficient safety protection structures and poor maneuverability. The rapid increase in their use has exacerbated the severity of crash-related fatalities and injuries. While some countries have implemented motorcycle restrictions to address this phenomenon, the causal effect of such policies on reducing the severity of injuries from accidents requires further research. This study employs a difference-in-differences approach to estimate the causal effect of motorcycle restrictions in China on the severity of road crash injuries. The results indicate a significant reduction in the likelihood of fatal or severe injuries occurring in crashes. Specifically, in regions with motorcycle restrictions, the probability of crashes resulting in fatal or severe injuries decreased by 2.9% on average. The results remain robust following placebo and falsification tests and excluding confounding effects. Heterogeneity analysis reveals that motorcycle restrictions primarily affect male drivers and drivers with urban hukou, with no significant differences in effects across drivers of different age groups. Overall, the findings indicate that motorcycle restrictions reduce the number of motorcycles on the road and decrease the probability of motorcycle drivers being at fault in crashes, signifying improved driver behavior and effectively lowering crash severity.