The invention discloses a steel plate pile and a support construction method for a foundation pit, wherein the steel plate pile comprises a steel plate, the steel plate is a strip-shaped and both endsof the steel plate are provided with a lock catch matching part, the steel plate is sunken inwards in the middle part of the vertical side face near the lock catch matching part to form a first groove along the height of the steel plate, which is vertical through, and whose cross section is T-shaped; an H-shaped steel, which comprises a web plate and a pair of wing plates which are vertically fixed on two sides of the H-shaped steel, the H-shaped steel includes a first wing plate and a second wing plate that are equal in width to the steel plate, and the H-shaped steel is matched with the inner side portion of the first groove, and the first wing plate is provided with a lock catch matching part at both ends, the vertical side of the outer side of the first wing plates sunken inwards to form a second groove, which is vertical through, and whose cross section is T-shaped; the second wing plate is clamped in the first groove, and the steel plate and the H-shaped steel are spliced to form an H-shaped steel sheet pile. The steel plate pile has the advantages of convenient construction, stable structure, and the like, and the steel plate pile can be widely applied to the technical field of steel sheet pile and foundation pit support construction method.