To ensure the time determinability of avionics applications based on Commercialofftheshelf (COTS) platform when accessing into AFDX (Avionics Full Duplex switched ethernet) networkan AFDX realtime flow middleware was designed.Using protocol conversion function based on filtering mechanism of intermediate driverthe middleware achieved communications between networks running with different protocol.Core code of frame sequence number (SN) insertion was described.A time scheduling algorithm which has the minimum jitter aimed at VLMultiplexing was proposedby distributing the sendingtime of visual links(VL)it is able to avoid contention on a rate limited physical link.Finallyexperiments were made to test the performance of AFDX realtime flow middleware and the results showed that:the middleware model achieved the format mutual conversion between TCP/ IP protocol and AFDX protocoland the time scheduling algorithm can decrease jitter caused by VL multiplex apparentlywhich ensured the time determinability of AFDX communication.