Since trust relationships are uncertain in open networks, it is necess ary to consider uncertainty when modeling trust.A novel trust model was proposed based on the cloud model, which was a useful tool to describe uncertain concept . Modeling trust with cloud, the proposed model could present trust degree and u ncertainty degree in a uniform form. Based on such a definition of trust, algori thms to compute propagated trust clouds and aggregated trust clouds were propose d, which were needed in the case of trust reasoning. With simulation experiment s, the proposed model were compared with the other two trust models. The results show the cloud-based trust model performs much better in open networks, especi ally in helping entities to cooperate with each other.