Abstract: To analyze the key performance indicators of cellular network communication and improve the reliability of communication systems, the user terminals uniformly distributed in cellular networks were analyzed. The random geometric modeling of user terminals in the downlink of ultra dense networks in 5G communication systems was conducted, and the situation of user equipment interfered by a base station outside the cell was analyzed. The probability density function of the user equipment′s signal-to-interference ratio was given, and the interruption probability based on the signal-to-interference ratio was calculated. The analysis results were verified by Monte Carlo simulation. The results show that the proposed interruption probability analysis method can be applied to the downlink communication scenario of heterogeneous cellular networks.