Based on the standard of IEC61850, the paper constructed the mathematical model of SMU (Substation Main Unit) processing delay by using the multi-server queuing theory, researched into the uncertainty delay caused by the emergency of queuing up phenomenon in the buffer of SMU to derive the analytical expression between buffer size, packet loss rate and delay, and proposed the method verified by examples, which can determine the buffering area suitable for packet loss rate and delay. The research overcomes the limitation in the previous studies based on the SMU of single CPU and constructs the model by taking SMU of multi CPU and IEDs as a whole, which can improve the delay certainty effectively and provides theoretical evidence for the buffering sizes design of communication module of SMU.