The invention discloses a concrete block closing device for a post-cast strip. The device comprises a left block closing device and a right block closing device on two sides of the post-cast strip, wherein the left and right block closing devices are same in the aspect of structure; the device further comprises an upper holder, a lower holder and a steel bar net rack mounted between the upper andlower holders; the upper and lower holders are the same in the aspect of structure, are mounted face to face, and comprise N interdependent holders connected end to end, or N independent holders connected end to end and a special holder; the lengths of the independent holders are twice as much as the distance between the floor reinforcements; the cross sections of the independent holders are isosceles trapezoids; the width of a mounting surface is smaller than or equal to 80 mm; and a skeleton groove and two inner grooves are formed in the mounting surface. A mounting construction method comprises the steps of off-site manufacturing the independent holders and the steel bar net rack, mounting the lower holder, the steel bar net rack and the upper holder in sequence, pouring floor concreteand pouring the post-cast strip. The block closing device is simple in structure, light in mass, convenient to store and carry, convenient to mount and high in construction efficiency, and is capableof effectively blocking the concrete and preventing grout leakage.