Biochar as an amendment has been widely applied to enhance crop productivity and improve soil quality. However, the effect of biochar-based fertilizer (BF) on the production capacity and economic benefits of open-field eggplant in the karst region remains unclear. A field experiment was carried out in the karst region of Southwest China from 2020 to 2021 to study the ameliorative roles of different application rates (1875, 2250, 2625, and 3000 kg ha⁻¹) of BF on the fresh yield, quality, fertilizer utilization, and economic benefits of fresh eggplant. The results show that BF increased the yield of fresh eggplant by 3.65–13.76% (2020) and 23.40–49.04% (2021) compared to the traditional fertilization practice (TFP). The application of BF reduced the nitrate content and increased the vitamin C (VC) and soluble sugar content of the fruits, which is beneficial for improving the quality of eggplant fruits. Meanwhile, the application of BF not only increased the nutrient uptake of the eggplant but also significantly improved the fertilizer utilization rates, especially the agronomic efficiency (AE) and recovery efficiency (RE). Moreover, BF could also significantly increase the output value and net income of fresh eggplant, which can help farmers increase their income. In conclusion, a BF application rate of 2544–2625 kg ha⁻¹ could be used to improve the yield, fertilizer efficiency, and economic benefits of open-field eggplant and is recommended for managing agricultural production in the karst region of Southwest China.