First input end of NAND Schmidt trigger receives input signal. After being connected to charging delay circuit in series, second input end is connected to the first input end. Output end of the NAND Schmidt trigger is in use for outputting control signal. The invention also discloses method for controlling flow direction automatically including following steps: in idle period of transmission, input signal keeps high level, automatic flow direction control device (AFDCD) outputs low level; when the input signal inputs a low level, AFDCD outputs high level instantly; when the input signal jumpsto high level from low level, AFDCD maintains high output level for momentary time; after the momentary time, AFDCD outputs low level. Baud rate of the disclosed AFDCD is 300-115200bps self-adapted. Based on input signal, the invention can control flow direction of RS-485/422 interface chip automatically.