The present invention relates to a synchronous hydraulic shear for continuous casting square billets. The synchronous hydraulic shear comprises a shearing mechanism, a rack and a walking device. The shearing mechanism comprises an upper cutter support, a lower cutter support, a support stand column, a bottom support seat, a first cutter shearing rail, a second cutter shearing rail, a first gap adjusting rail, a second gap adjusting rail, an upper cutter, a lower cutter, a shearing hydraulic cylinder, a gap adjusting hydraulic cylinder and a gap adjusting hydraulic cylinder support plate. When the gap between the cutters needs to be adjusted, a piston rod of the gap adjusting hydraulic cylinder stretches and retracts to drive the upper cutter to move in the first gap adjusting rail and the second gap adjusting rail, thus to increase or decrease the gap between the upper cutter and the lower cutter, and enable the gap to be optimal. A double-acting double-piston rod hydraulic cylinder is selected as a walking hydraulic cylinder, so the operating stability of the hydraulic shear is improved. The gap between the cutters can be adjusted in real time, deformations of the sheared square billets are small, and synchronous operation is stable.