Through directed bipartite graph and the method of stochastic process, based on the supply chain for many suppliers and demanders, a reliable stochastic model is set up under the comprehensive consideration of the effects of time as well as internal and external factors, meanwhile, the case of the unique identity of suppliers and demanders is also confirmed. Moreover, through the discussion of the model, a unified formula for calculating the failure probability of the suppliers and demanders is also given, and the model is popularized in other cases, namely, when the identity of suppliers and demanders is not uniquely confirmed.