The invention relates to a dewatering earth-taking construction method for a foundation pit, in particular to a dewatering earth-taking construction method for a foundation pit on water, which is characterized in that the foundation pit is divided into n layers from top down for dewatering earth-taking construction in sequence, a steel bracing is constructed on each layer, and finally excavation is carried out till the bottom of the foundation pit; wet earth-taking method is adopted for earth taking from the first layer to the n-1 layer, or concretely speaking, a sludge pump, a keg float and a high pressure water gun form a water and sludge suction group; the high pressure water gun jets high pressure water flow to earth to make the earth become sludge which is sucked by the sludge pump, discharged into a sludge barge by an outflow pipe and delivered to an appointed area to be unloaded, and the dewatering and sludge suction are carried out at the same time; dry earth-taking method is adopted on the n layer.