The invention relates to an online main pump flushing device and method for a middle booster pump station. The online main pump flushing device comprises an upper pump station (7), a main pump (1) of a middle pump station, and a lower pump station (8), wherein the upper pump station (7) and the main pump (1) of the middle pump station as well as the main pump (1) of the middle pump station and the lower pump station (8) are respectively connected with each other in series through main ore pulp pipelines (2) at two ends; a bypass ore pulp pipeline (3) is connected in parallel between an inlet and an outlet of the main pump (1) of the middle pump station; the bypass ore pulp pipeline (3) directly conveys ore pulp conveyed by the upper pump station (7) to the lower pump station (8) when the main pump (1) of the middle pump station shuts down; and the main pump (1) of the middle pump station is connected with a water circulation pump (5) in a process pool (4) through a flushing water pipe (6).