A hydraulic locking mechanism for multiple extrusion heads of an extruder, has two Y-shaped clamping arms, six wedges, six pushing bars, six oil cylinder brackets, six oil cylinders, and six locking holes. A pair of locking holes is formed on both sides of an upper portion and a lower portion of the barrel and the extrusion head thereof, respectively. The wedge is disposed on inner wall of the locking hole, and a locking hole thereof has a sectional area gradually decreasing from the outside to the inside. The clamping arms are disposed on both sides of the barrel and the extrusion head via fastening devices. A through hole for the pushing bar is disposed on a branch of each of the clamping arm and corresponds to six locking holes. A piston rod of the oil cylinder is connected to a rear end of the pushing bar, the six oil cylinders and the six pushing bars are respectively fixed on six branches of the two clamping arms via six brackets of an oil cylinder. The pushing bar is capable of moving along the through hole for the pushing bar as being driven by the oil cylinder, and a front end thereof is inserted in the locking hole and penetrably disposed in the through hole for the pushing bar. A slant is formed on the outer side on a front end of the pushing bar.