A method of detecting master/slave response time-out under continuous packet format communications protocol, which calculates the time required for the slave device to respond to a Modbus request subject to Modbus TCP/UDP protocol. The method is to continuously send Modbus requests to a slave device through a detection device and to record each Modbus request sent time, and to have the slave device provide to the detection device a response for each Modbus request. By means of calculating the precise response time-out from the response time-outs which are gotten from the slave device responds to a predetermined number of Modbus requests, the user or manager can determine the response time-out required for the slave device precisely so as to give an EXECUTE instruction or command at the accurate time point.