Based on the finite-rate numerical simulation coupled with detailed reaction mechanisms of nitrogen, the strong turbulence-chemistry interaction in flameless combustion can be considered, and the prediction accuracy of key combustion parameters and NO formation can be improved. A reasonable fast calculation method was builded that can greatly accelerate the calculation without significantly sacrificing the simulation accuracy. A coupled calculation method of In-Situ Adaptive Tabulation and Dynamic Adaptive Chemistry was developed based on the high-fidelity numerical simulations of CH4/H2 flameless combustion with a hot co-flow, then the nitrogen conversion mechanisms of flameless combustion was further analyzed. It is found that the coupled calculation method can ensure the higher accuracy required for the simulations and double the calculation speed compared with a single method. It can be further optimized with a reasonable error threshold and the Dynamic Adaptive Chemistry threshold 10-2 performs best overall. The coupled calculation method has a larger simplification space for high-complexity mechanisms, and a nearly 10.8 times calculation acceleration is obtained with one detailed reaction mechanism of nitrogen containing 151 species and 1 397 reactions. Based on the experimental validation and the nitrogen conversion path analysis, the NO formation of CH4/H2 flameless combustion with a hot co-flow mainly depends on the NNH path and the N2O intermediate path, while thermal-NO and prompt-NO can be ignored.Compared with the In-Situ Adaptive Tabulation, the nitrogen conversion path analysis can be properly simplified and the main NO paths can be retained with the coupled calculation method.