The invention discloses an internal corner connector of a foundation pit supporting system. The internal corner connector comprises a support plate which is a whole piece of a steel plate, wherein the support plate comprises a main plate body which integrally takes a shape of a rectangle and a quadrilateral subplate body which is integrally connected to the one third position of a long side of the main plate body; the long side, away from one side of the subplate body, of the main plate body of the support plate is vertically welded with a rear connecting plate, the short side, adjacent to a first connecting side, of the main plate body is vertically welded with a square flange plate, the other short side of the main plate body is vertically welded with a third side connecting plate, the first connecting side is vertically welded with a first side connecting plate, and a second connecting side is vertically welded with the flange plate; a third connecting side is vertically welded with the flange plate, a fourth connecting side is vertically welded with a second side connecting plate, and the other side of the subplate body is vertically welded with a front connecting plate. The foundation pit supporting system is simultaneously connected with two-layer steel beam type section steel on two sides of an internal corner of a foundation pit through the internal corner connector, and the internal corner connector is convenient to mount and dismantle.