In the recent years, some domestic overflow and blowout events have shown that we can not apply conventional methods to successful well killing under some certain conditions, therefore, bullheading as an unconventional well killing can be considered. Bullheading by pumping kill fluid into wellbore makes the wellbore gas compressed so that bottomhole pressure exceeds formation pressure and gas leaks off to the formation. In this study, applicable conditions of bullheading have been discussed. Based on gas compressibility law and gas-liquid flow behavoir, bullheading operation is divided into three stages, including gas compression stage, gas seepage stage and gas-liquid seepage stage. The mathematical model for each stage is formulated to predict and interpret well killing mechanism. The calculated results showed good agreement with field case data. This paper might serve as an operational guideline during bullheading.