In this paper, we propose an automatic, self-adaptive trust model for component-based Service-Oriented Architecture that is based on Bayesian model. The focus of this model is the "quantitative trust on action". Both the computation model and its updating mechanism are given. Through our implementation of this model using Monte-Carlo algorithm, we show the feasibility and practicability of our proposal as well as its ability to rectify any unfair advertisement and to track the capability of highly dynamic service.