To reduce the unnecessary overhead of data source authentication in static security mechanism and defend the On-off attack in trust threshold mechanism, an adaptive security mechanism based on trust was proposed in the Internet of Things (IoT). Firstly, the trust evaluation model was built according to node behavior in information interaction, further the measure method for total trust value of nodes was given. Then, for the nodes whose total trust values were higher than the trust threshold, the trust-based adaptive detection algorithm was used to detect the changes of the total trust values of these nodes in real time. Finally, the relay nodes determined whether to authenticate the received message according to the returned result of adaptive detection algorithm. The simulation results and analysis show that the proposed mechanism reduces the energy overhead of relay nodes, and plays a better role in defense against On-off attacks in IoT.