Abstract: The aim of this study was to determine the optimal amount of straw biochar added to the dry-farming baby cabbage in the mountainous area of southern Ningxia, and to provide a theoretical basis for soil fertilization and safe production of vegetables in the rain-fed vegetable fields in the mountainous area of southern Ningxia. Starting from 2020, by using the baby cabbage as the tested crop, the randomized block experiment design was adopted, and the field experiments were conducted for two consecutive years with no fertilizer and biochar as a complete blank contrast (CK), in order to study different biochar application rates (0 t·hm⁻², 10 t·hm⁻², 20 t·hm⁻², and 30 t·hm⁻²) combined with the application of nitrogen fertilizer on the growth, photosynthesis, yield and quality, water and fertilizer utilization efficiency, soil physicochemical properties and soil enzyme activities of the dry-farming baby cabbage. The results showed that: the application of biochar or not had no significant effect on the root system, but the amount of precipitation during the growth period could affect the growth and yield of the whole baby cabbage in dry farming. The cumulative value of net photosynthetic rate in the treatment of adding biochar was higher than that without adding biochar. The addition of 30 t·hm⁻² biochar and aging for one year could significantly increase the activities of sucrase, alkaline phosphatase and urease by more than 2.12%, 1.19% and 13.68%, respectively, increase the yield by more than 8.11%, save water by more than 9.84%, and increase the income by more than 4316.85 yuan·hm⁻². Therefore, comprehensively considering the quality index, production cost, economic benefit, water and fertilizer utilization efficiency and the aging effect of biochar, it was determined that under the condition of combined application of nitrogen fertilizer, the addition of 30 t·hm⁻² biochar was the optimum addition amount of straw biochar for the rain-fed dry-farming baby cabbage in the mountainous area of southern Ningxia, which could achieve water saving and yield & efficiency increasing to a certain extent. At the same time, it was concluded that the greater the addition amount of biochar, the smaller the effect of aging on it was.