Abstract: To improve security of communication data in WSN (Wireless Sensor Networks), a semantic security algorithm based on a changing mapping table was presented. Considering of threshold-sensitive characteristics of WSN application, a data collection method with variable precision was used, i.e. high collection precision close to threshold and low collection precision far from threshold. Then a mapping table was introduced which defined the mapping way of identification and original semantic. The encryption in the nodes and decrypt in the gateway were depended on the mapping table, which kept synchronization between the gateway and the node during the communication. To enhance security, the mapping table was continuously updated based on a random changing factor. And some improved random number generators were suggested in order to improve the nonlinear feature of mapping table conversion. The time complexity analysis, space complexity analysis and energy consumption analysis were given. Finally, some experiments were done with Micaz sensor nodes and the simulation results and analysis were given.