The invention discloses a construction method for a foundation base depressurization and drainage type cofferdam with Z-shaped underpinning support steel sheet piles. The method comprises the steps that the perpendicularity of steel casings and the riverbed surface are measured; Z-shaped underpinning brackets are manufactured and welded; inner supports are machined and installed; single-layer inner supports are lifted and laid down; steel sheet piles are transported to a construction site and are struck; water is pumped, and excavation is carried out; a seepage diversion layer is laid; a foundation pit is monitored; concrete pouring is carried out; a support system is converted; and pier studs are constructed, and the steel sheet piles are disassembled. According to the method, the thickness of bottom-sealing concrete is decreased, the foundation base is subjected to seepage diversion and depressurization so that the thickness of the bottom-sealing concrete can be greatly decreased, meanwhile, the embedment length of steel sheet pile cofferdams can be decreased, and dry bottom sealing of the cofferdams can be realized; and the excavation amount and excavation depth can be decreased by decreasing the thickness of the bottom-sealing concrete, the damage disturbance on the foundation base can be reduced, and therefore the cofferdams with the same embedment depth tend to be more stable.