The invention discloses a pile bottom preloading device and method for a steel pipe pile. The pile bottom preloading device comprises a mudjacking pipe frame, a plurality of sets of mudjacking pipe loops, a mudjacking sleeve piece and a grouting flow distributor, wherein the mudjacking pipe frame is arranged in the steel pipe pile; the mudjacking pipe loops are fixedly arranged on the mudjacking pipe frame, and a pressure gauge and a first ball valve are arranged at an outlet of each set of mudjacking pipe loop; the bottoms of the mudjacking pipe loops are sleeved with the mudjacking sleeve piece, and vertically-downward holes are formed in the mudjacking sleeve piece; and the grouting flow distributor comprises a grout inlet and a plurality of grout outlets, a second ball valve is mounted at each grout outlet, the grout outlets are in one-to-one correspondence with the mudjacking pipe loops and are connected with the mudjacking pipe loops, and the grout inlet is connected with the mudjacking machine. According to the pile bottom preloading device and method for the steel pipe pile, by injecting cement grout into the space between an intra-pile soil plug and a concrete plug, the actual ultimate end resistance of the steel pipe pile in a stratum of an ultra-fine compact silt structure can reach the geologically-explored end resistance, and the pile end ultimate bearing capacity can be greatly improved.