The utility model discloses a cast -in -place concrete slab is forming device for stake, including steel form and the shoe of flap valve formula, the center of steel form is provided with the cavity of pouring that the cross -section is the rectangle, and the top or the lateral wall top of steel form are provided with the concrete placement entry, the shoe of flap valve formula sets up in the bottom of steel form, and the shoe of flap valve formula includes two blocks of triangle -shaped boards and two flap valve baffles, every block of triangle -shaped board all sets up with the steel form isintegrative, and every flap valve baffle is all articulated mutually bottom the steel form, two flap valve baffle one side bottoms in opposite directions all are provided with oblique point and magnet, when two oblique points adsorbed mutually, two flap valve baffles were laminated with two blocks of triangle -shaped boards mutually to enclose close form one with the sealed pouring cavity that the cavity is linked together pours. The utility model discloses enable the sheet pile and carry out the pore -forming at the construction field (site) and fill the shaping, saved complicated intermediate link, reduce cost, the buried depth of sheet pile also can show to be increased. In addition, ambient noise can also be reduced, the efficiency of construction is improved.