The main errors of the SCADA system stem from RTU's asynchronous measurement and the delay in data transmission. A synchronous measurement unit（SMU） is designed based on GPS and time synchronization on the Ethernet. It can realize data acquisition and real-time transmission simultaneously， just like PMU but costs less. It is composed mainly of the signals transformation circuit， man-machine interface（MMI）， control module and communication circuit. SMU is capable of providing highly accurate data with unified time label by such processes as clock synchronizing， synchronous sampling and synchronous time-marking. The design can be extensively configured for both synchronous measurement capability and low-cost.