The invention relates to a double-lock-catch flat steel pipe pile used for foundation pit support. The double-lock-catch flat steel pipe pile used for foundation pit support is characterized in that the support structure is composed of flat steel pipe pile bodies, inner ribs, inclined struts, lock catches and water resisting bodies; three steel plate inner ribs are arranged inside each flat steel pipe pile body at equal intervals; the two ends of each flat steel pipe pile body are arc; the flat steel pipe pile bodies are connected through double lock catches; the lock catches and the flat steel pipe pile bodies are welded through end points and are enhanced and fixed through the steel plate inclined struts on the two sides; each lock catch part comprises a socket steel plate, a cylinder insertion tenon and a cylinder rabbet; the cylinder insertion tenons and the cylinder rabbets are in a hollow state; the water resisting bodies are water resisting body materials filling the cavity formed by the socket steel plates, the insertion tenons and the rabbets. The double-lock-catch flat steel pipe pile used for foundation pit support can completely eradicate piping and flowing oil disasters, is simple in structure, low in manufacturing cost and convenient to construct, can be prefabricated in a factory and installed on a site, and is capable of being recycled, good in environmental protection benefits and short in construction time limit.