During ultra-deepwater drilling operations for oil and gas, the choice of the pump displacement is an important problem that concerns safety and economic results of offshore drilling operations. In addition to three conventional constraint conditions, another constraint condition has been established to calculate the critical pump displacement. The collapse pressure, pore pressure, fracture pressure, and sufficient safety margin are taken into account in the fourth constraint condition. The method is presented in detail. Based on the equivalent circulating density calculation model in ultra-deepwater drilling, the formation pressure data, and the safety coefficient, the result of critical pump displacement can be obtained. The calculation procedure of the critical pump displacement is presented through an example. According to the analysis, the safety margin is one of the critical limitations for pump displacement and the pump displacement determination is dependent on all of these four constraint conditions.