This paper investigated the theories and methods of prestack joint inversion by utilizing PP and PS converted wave data sets. Actual multi-component seismic data from the oilfield were also used to delineate the deep buried volcanic gas reservoir based on the studied methods. Well log data show that the gas reservoir prediction results are consistent well with the well data, suggesting that prestack PP-PS joint inversion can avoid the multiple solutions of lithologic parameters caused by only using PP compression wave, and can improve the accuracy of gas reservoir prediction.