The protection relays such as transformer relay, busbar relay and so on are installed outdoors which adopt the distributed architecture. And the ring network communication mode is adopted among the distributed slaves. Testing ring network messages is difficult. A new test device is designed and developed, which can fulfill function test of ring network. The device adopts dual central processing unit(CPU) architecture. The master CPU is responsible for testing process management. The slave CPU is responsible for testing the data exchange of ring network. The device can build the ring network messages by configuration file. The devide can finish the following test function, including delay characteristics, data consistency, forwarding characteristics and abnormal message simulation. The reliability and real-time in the actual transmission process of ring network messages can be checked and it can be taken as supplement for the ring network performance test. One device can fulfill function test and performance test of all distributed slaves. The device has been used on-site widely and provides good test method for the protection relays installed outdoors.