Abstract: Researches were made on optimization design of a main circulating pump by establishing a calculation model based on self-developed improved complex shape algorithm to minimize its volume under the premise of meeting the given restraint conditions. In view of the fact that it is one of the essentials to accurately calculate the lowering speed of reactor coolant flow in reactor safety analysis, an assessment was conducted on the flow coastdown transient process of optimal results, under coastdown conditions before startup of the passive residual heat removal system, with the transient calculation model of main circulating pump. Results show that significant effects can be obtained for the main circulating pump through above optimization, and the coolant pumping time of optimization model is close to the prototype, with safety not worse than the prototype.