In this paper, a reservoir simulation study was conducted for the characterization and prediction of gas breakthrough during the development of cyclic steam and gas stimulation (CSGS) for a horizontal well. A new concept named the gas breakthrough coefficient (GBC) was proposed to characterize the gas breakthrough degree quantitatively, and a regression model and two calibration curves were established to predict the gas breakthrough degree. The method of foam plugging to inhibit gas breakthrough was also discussed. It was found that the gas breakthrough degree could be well characterized by the GBC and distinguished as four types: weak, moderate, strong, and severe. The regression model and calibration curves could also be used to predict the gas breakthrough degree under different reservoir and development conditions. Foam plugging was found to be effective to inhibit gas breakthrough when the gas breakthrough degree was moderate or strong.