Wireless　sensor　network　is　a　key　technology　in　Internet　of　Things.　However,　due　to　the　large　number　of　sensor　nodes　and　limited　security　capability,　aging　nodes　and　malicious　nodes　increase.　In　order　to　detect　the　untrusted　nodes　in　the　network　quickly　and　effectively　and　ensure　the　reliable　operation　of　the　network,　this　paper　proposes　a　dynamic　network　security　mechanism.　Firstly,　the　direct　trust　value　of　the　node　is　established　based　on　its　behavior　in　the　regional　information　interaction.　Then,　the　comprehensive　trust　value　is　calculated　according　to　the　trust　recommendation　value　and　energy　evaluation　value　of　other　high-trust　nodes.　Finally,　node　reliability　and　management　nodes　are　updated　periodically.　Malicious　nodes　are　detected　and　isolated　according　to　the　credibility　to　ensure　the　dynamic,　safe,　and　reliable　operation　of　the　network.　Simulation　results　and　analysis　show　that　the　node　trust　value　calculated　by　this　mechanism　can　reflect　its　credibility　truly　and　accurately.　In　terms　of　reliable　network　operation,　the　mechanism　can　effectively　detect　malicious　nodes,　with　higher　detection　rate,　avoid　the　risk　of　malicious　nodes　as　management　nodes,　reduce　the　energy　consumption　of　nodes,　and　also　play　a　defensive　role　in　DOS　attacks　in　wireless　sensor　networks.