An automatic opening and closing device for a mold assembly includes a base, a pivot seat, a first extensible unit, an upper mold mounting unit, two shafts, a second extensible unit, a left arm and a right arm. The first extensible unit can move the upper mold mounting unit downward to clamp or release the mold, and the second extensible unit is used to move the pivot seat between two positions. When the pivot seat is in a first position, the upper mold mounting unit can be lowered down to secure or release the mold by the first extensible unit, when in the second position, the upper mold mounting unit and the mold can be rotated an angle, to perform opening or closing of the mold assembly.