In recent years, trust has emerged with the development of cloud computing. It is a critical step to select a trusted cloud provider before the service begins, which is related to the interests of cloud consumers themselves and the quality of the service. A SLA trust model based on behavior evaluation is proposed in this paper. User's subjective evaluations are abandoned, the provider who is trusted and meets the demand for cloud consumers is selected according to the transaction history (mainly the parameter vectors formed during the transaction process) between cloud providers and cloud consumers and trust value of cloud providers before the service starts. In the service process, using iterative methods dynamically updates the trust value based on the fulfillment of SLA parameters. At the same time, the time factor is taken into account, so that the trust value is more reasonable. Experiments show that the model is able to select trusted provider to trade according to the demands of cloud consumers, dynamically updates the trust value and the trust value of malicious providers can be suppressed.