This paper focuses on the preparation of environmentally friendly biomass carbon-based solid acid catalysts from black soldier fly larvae (BSFL) shells. The effective catalysts were prepared by incomplete carbonization of BSFL shells and then the p-toluenesulfonic acid (PTSA) groups were introduced to obtain catalysts. The synthesized composite catalysts were characterized by XPS, FT-IR, SEM, XRD, and BET analysis to reveal the physiochemical properties. Meanwhile, the effects of various reaction conditions on the yield of biodiesel with oleic acid (OA) and methanol were studied in detail. The results showed that the conversion rate reached 93.2% under the optimal conditions in this paper: carbonization temperature = 400 °C, carbonization time = 2 h, sulfonation temperature = 100 °C, and reaction time = 4 h, respectively. Furthermore, the stability and reusability of the prepared catalysts were also demonstrated. At last, the possible catalytic mechanism of the prepared catalyst was comprehensively described. Moreover, the results showed that the synthesized biomass-based solid acid catalyst under these conditions had good catalytic activity.