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Underground bio-methanation (UBM) technology offers a promising approach for carbon utilization and sequestration, as well as large-scale underground energy storage. This study is the first to apply machine learning methods to forecast CO2 conversion ratios in UBM, using extensive numerical simulation data. Nine key input parameters were sampled via Latin Hypercube Sampling, generating 3,000 simulation cases used to train and evaluate three classical models: Random Forest (RF), Least Squares Boosting (LSBoost), and Feedforward Neural Network (FNN). The results show that the FNN model yields the highest predictive accuracy, as indicated by the highest coefficient of determination (R2) on the test set, and the lowest mean absolute error (MAE) and root mean square error (RMSE) values. Further analysis of varying sample sizes (1500 to 3000) using FNN demonstrates that increasing the training data significantly improves model performance, with a notable enhancement at 3000 samples (test R2 = 0.990, MAE = 0.017, RMSE = 0.029). These findings underscore the effectiveness of FNN in rapidly and accurately predicting CO2 conversion ratios, thereby supporting optimized site selection and operational management in UBM processes.