Underbanlanced drilling (UBD) technology is widely used in practice because it has many advantages such as improving drilling speed and discovering and protecting reservoir timeliness. However, after the formation gas enters into wellbore annulus, the rule of gas-liquid two-phase flow is complicate, because different gas-liquid flow pattern has different throughput capacity, as a result, the rule of cuttings transport is varied. Firstly, the essay introduces cuttings transport mathematical model; Secondly, it introduces the model of distinguishing different multiple phase flow patterns and the mathematical model of calculating annulus pressure; Finally, it takes some well for example and formulates calculator program, analyzes drilling parameters and optimizes drilling parameters. The results coincide with field parameters.