This study conducted a pot experiment using karst calcareous soil from Guizhou province, focusing on the effects of biochar-based fertilizer (made from rice-husk-derived biochar, pig manure compost, and NPK fertilizer) on the contents of various types of soil phosphorus, soil alkaline phosphatase activity, and biomass of black locust seedlings. Ten treatments were set up:CK, M, F, MF, RH1MF, RH2MF, RH4MF, RH8MF, RH4M,and RH4F (CK is control, M is compost, F is NPK fertilizer, RH is rice-husk-derived biochar and the number is the amount of biochar in the treatment based on the ratio of biochar to soil). Our results showed that the application of biochar-based fertilizer could significantly increase the contents of soil total phosphorus, available phosphorus, organic phosphorus, microbial biomass phosphorus, and seedling biomass, with the strongest effects with the highest biochar application rate (RH8MF). The above 4 different types of phosphorus showed highly significant positive correlations (P < 0.01) with each other. The soil alkaline phosphatase activity was highest with the treatments with the medium biochar application rates (RH2MF and RH4MF), at 82.7% and 63.4% higher than CK respectively. In conclusion, biochar-based fertilizer application, especially at higher biochar addition rates, significantly increased phosphorus content of karst calcareous soil and improved the low-content status, so as to improve the effects of ecological restoration of young artificial woodlands in karst areas.