Automatic fixture replacement mechanism for machine tool, wherein includes multiple sets of fixtures for installing work pieces to be processed and a movable manipulator for loading fixture; the movable manipulator is installed on a two-dimensional movable mechanism; the movable manipulator is provided with a pull nail that is fastened to the clamp and the fixture is provided with a pin hole matching with the pin. For batch processing of a large number of work pieces with multiple fixtures, the use of movable manipulator to automatically install and unload fixtures can greatly improve the efficiency of processing. The structure of the mechanism and the operation are simple so that it is reliable and easy to be implemented. Hence, it can be widely used in various types of CNC machining centers.