Abstract: Nowadays, aerated underbalanced drilling becomes one of the most commonly used techniques in coalbed methane drilling. It is difficult to satisfy the rock cuttings portability requirement and control the stability of bottom hole pressure, while controlling the gas injection pressure, rate, and volume. The pressure drop performance is obtained at the corresponding cave through numerical simulation by FLUENT software under different gas injection pressure, rate and volume. So a reasonable set of technical parameters are selected, which can guarantee the expected flow type to fulfill the rock portability requirements, and stabilize the bottom hole pressure during aerated underbalanced drilling. An analyzing model is built up through the real-time parameters from one well, and the gas injection pressure, rate and volume are optimized, following with the parameter varying rules. The results have some guidance to ensure drilling safety and optimize drilling operations.