Typical slewing valve is a three-position six-way proportional valve based on bypass throttle principle, whose output flow rate and opening dead zone of the main valve port are both affected by the load. Thus, it has poor performances in dynamic control of varying loads. This article presents a novel slewing valve based on bypass pressure-compensation principle, which has much better performance in dynamic control of varying loads and even for large inertia loads. A pressure-compensated valve is added to connect the out ports of the main valve port and the bypass port to keep the pressure differences at the main valve port and the bypass port in same. As a result, the flow distribution ratio of these two ports keeps stable for a certain spool position, which can avoid the output flow rate fluctuation despite the varying loads. In addition, the opening dead zone of the main valve port is very small and keeps almost unchanged. These advantages make the proposed valve to control large inertia load with great stability. In the article, a mathematical model formulating the dynamic performance of the valve is further established to provide guide for the optimization of the parameters, including the shapes and orifice areas of the main valve port and bypass port, the stiffness of the spring controlling spool motion, and so on. A prototype valve was manufactured based on the presented method. A series of tests on experiment bench and real crane validate its great performances on flow rate and dead zone stabilities as well as fast dynamic response.