Abstract: This paper introduces the design of multi-state supervisory field data acquisition system with PHOENIX RS422 /485 PRO module as the core for freeway tunnel.The field data are acquired and exchanged with computer and realized real-time display by using programmable logic controller(PLC).In the PCWORX, corresponding function blocks is used for setting communication protocol and other communications parameters.Through in-situ test, it is demonstrated that the module has the features of flexibility and practicability in the multi-state supervisory system for expressway tunnels.