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Nitrogen is an essential nutrient for the growth of rice. In order to increase yield, excessive application of nitrogen fertilizer often leads to low nitrogen utilization efficiency and also damages soil quality. Reducing the application of nitrogen fertilizer will lead to a decrease in rice yield. It is worth exploring whether biochar, as a soil amendment, can compensate for the negative effects of nitrogen reduction. A pot experiment was conducted to investigate the compensatory effects and mechanisms of biochar on soil fertility and rice yield under four conditions: conventional nitrogen application (nitrogen fertilizer application rate of 110 kg/hm2, CK), 10% nitrogen reduction (C1), and 2.5% (C2) and 5% (C3) concentration biochar application under 10% nitrogen reduction. The results showed that the nitrogen fertilizer reduced by 10% had no significant effect on soil comprehensive fertility. Applying biochar could effectively improve soil fertility, and 5% concentration of biochar had better effect on soil fertility than that of 2.5% concentration. Reducing nitrogen by 10% significantly reduced rice yield, and the application of biochar can compensate for the reduction caused by nitrogen reduction to some extent. Among them, the compensation effect of 2.5% concentration of biochar on rice yield was better than that of 5% concentration, but it had not yet returned to the conventional nitrogen application level. Exploring the driving factors of rice yield under nitrogen reduction and biochar application conditions by using structural equations, the results showed that reducing nitrogen fertilizer combined with biochar can directly or indirectly affect rice yield by changing soil pH value, total potassium, ammonium nitrogen, and nitrate nitrogen content. Among them, ammonium nitrogen and nitrate nitrogen content were the main regulatory factors, which not only directly affected rice yield, but also indirectly affected the formation of rice yield by regulating the number of grains per panicle, effective panicle, and thousand grain weight; pH value and total potassium content had negative effects on rice yield. The research result can provide scientific basis for the reduction of fertilizer and efficient utilization of biochar in rice cultivation in the black soil area of Northeast China.