The present disclosure provides a method and a device for detecting an external compensation line and a display module. The method includes steps of: within a resetting time period of each detection stage, applying a resetting voltage to the external compensation line and entering a detection time period after a resetting duration; and within the detection time period of each detection stage, controlling the external compensation line to be in a floating state, applying a data voltage to a data line, applying a power source voltage to a power source voltage input end, applying a data write-in control voltage to a data write-in control end, applying an external compensation control voltage to an external compensation control end, detecting a voltage across the external compensation line after a detection duration, and determining whether or not there is a short circuit for the external compensation line in accordance with the voltage across the external compensation line.