Biochar stability determines the effectiveness of biochar's functions such as carbon sequestration, soil structure improvement, soil fertility enhancement and soil pollution remediation. However, a fast method for accurately predicting biochar long-term stability in soil remains elusive. Here, firstly, an incubation experiment was conducted on mineralization dynamics of different 13C-labelled biochars over 368 days to explore their actual mineralization in soils and establish their mineralization model. Thereafter, ten treatments of fast chemical oxidation methods using K2Cr2O7 (0.1 M) with different H+ concentrations and oxidation times were applied to the biochars to reveal which method best matches the mineralization of biochar in soils. Results showed that the percentage of biochar‑carbon oxidized by the solution containing 0.1 M K2Cr2O7 and 0.2 M H+ at 100 °C for 2 h was in accordance with the one that potentially would be mineralized in soils at a 100-year scale (R2 > 0.99; REMS = 2.53; RD = 15.3). The results provided a chemical oxidation method that was robust, effective, low cost and highly available for measuring the long-term stability of biochar in soils. Display omitted •The fraction of biochar-carbon oxidized by a solution with 0.1 M K2Cr2O7 and 0.2 M H+ at 100 °C for 2 hours, was very closely correlated (R2> 0.99) with the one that potentially would be mineralized in soils at a 100-year scale.•H/C and O/C were not accurate for predicting stability of biochar in soil.•Mineralization rate of biochar was more sensitive to inherent stability of biochar than to soil type, especially for biochars produced at above 300 °C.