Zanthoxylum bungeanum seed oil (ZSO) cannot be widely consumed because it contains high free fatty acids with an acid value of 73.75 mg KOH per g. Herein, a solid acid catalyst was prepared from ZSO via a carbonization–sulfonation method using concentrated sulfuric acid. The prepared catalyst was characterized by FT-IR, SEM, XPS, XRD, N₂ physisorption, and Boehm titration methods. The prepared solid acid was employed as a catalyst to investigate its performance during the simultaneous esterification and transesterification of ZSO in a one-pot process to produce biodiesel. The optimum reaction parameters for obtaining biodiesel in >95% yield using the abovementioned reaction were as follows: 30 : 1 methanol-to-oil molar ratio, a catalyst loading of 8.0 wt%, a reaction temperature of 140 °C, and reaction time of 4.0 h.