As analog-input merging unit is widely used in intelligent substation, its time performance whether meets the requirements of testing specification is directly related to the synchronization of sampling data, thus may affect the operation of measurement and protection devices in intelligent substation. Nowadays there are few studies on time performance of analog-input merging unit and few relatively comprehensive testing system. This paper analyzes the meaning of time performance parameters and its error sources according to the requirements for time performance of analog-input merging unit in testing specification, proposes some test methods aiming for different time parameters and different synchronization mode, develops a testing system for time performance parameters. Paper also verifies the accuracy of time measurement based on TDC, the accuracy of phase-difference measurement and the function of SV message analysis. Results show that testing system is in line with the requirement of “one third rule” and meets the demand of time performance testing.