The randomness of communication requirements and communication capabilities is the main reason that causes congestion as well as randomness of communication system service efficiency. Based on the analysis on effect of random factor on communication system requirements as well as random factors, such as interference, weather, accidents, on communication capabilities, the paper analyzed on randomness of communication requirements and communication capabilities and established functional function model of communication channel. Referring to reliability theory, the channel unblocked reliability index was defined. Through evaluation on congestion probability, the reliability of communication system running status was also analyzed. The research can provide reliability reference to design and planning of communication system.