IEEE 1588 synchronous clock realized with TCP/IP is used for communication networks timing in substations. The path time-delay error of synchronous message is caused by the communication network transmission congestion. The precision error of IEEE 1588 clock synchronization is analyzed， and the correction methods of path time-delay error of synchronous message are proposed with the diffserv-based packets scheduling model. The bandwidth adjustment factor， pressing degrees mechanism and the time-delay of the synchronous message is determined through setting of network node service message priority. Then the correction methods of oscillation frequency of clock generator are presented. The IEEE 1588 synchronization clock error is thus corrected. Finally， the high precision network clock hardware platform is built. Experiment results show that the clock has the synchronization precision of nanosecond， and it can satisfy the time synchronization precision requirement of smart substations of IEC 61850 Std.