The embodiment of the invention provides a method and device for determining the coupler slack after a heavy haul train brakes, the heavy haul train and a storage medium, and belongs to the field of rail transit. The method comprises the steps that the initial slack of each pair of couplers before the heavy haul train brakes is determined; the variation of the slack of each pair of couplers in thebraking process of the heavy haul train is determined; and the actual slack between each pair of couplers after the heavy haul train brakes is calculated according to the initial slack and the variation.