Short circuit failure is a common and harmful fault of the RS485 serial bus, which easily brings great damages to the serial bus controlling chips. Generally, detectable packets are sent to determine whether the RS485 serial bus works well or not. However, this method responds slowly and can not avoid damaging the relevant interface circuit. A novel detection technique for short circuit failure based on current detection is presented in this paper. It can detect the short circuit failure of the branch circuit online, and can shield the branch fault point immediately, which prevents short circuit failure from damaging the systems further.