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Intensifying nitric acid-type acid rain (NAR) poses a serious threat to terrestrial ecosystem functioning. Biochar (BC), as a soil amendment, has the potential to mitigate soil degradation caused by acid deposition. However, under severe NAR stress, the mechanisms by which BC affects soil fungal communities in plantations remain largely unexplored. In this study, a field experiment simulating severe NAR was conducted in a Quercus acutissima plantation. Two BC types, including pyrochar (PC) and hydrochar (HC), were applied to the soil to examine their effects on soil fungal community composition and diversity. The results showed that the PC application significantly increased the relative abundance of Basidiomycota in soil by 20.92%, whereas HC significantly decreased it by 57.83% under severe NAR stress. Linear discriminant analysis Effect Size results suggest that PC and HC addition promoted the enrichment of specific fungal taxa, such as Amanitaceae, Penicillium, and Strophariaceae. Additionally, PC and HC increased the soil fungal Shannon index by 19.81% and 42.45%, respectively, under severe NAR stress. Furthermore, redundancy analysis revealed that changes in soil fungal community structure induced by BC application under severe NAR were mainly driven by soil microbial biomass. Our findings demonstrate that HC is more effective than PC in alleviating severe NAR stress on soil fungal communities in plantations, providing a scientific basis for the resource utilization of BC and the ecological restoration of plantations in NAR-affected regions.