The invention provides a clamp used for machining a glass die and belongs to the technical field of tool clamps. The clamp comprises a base, a die joint face limiting base and a pair of die outer circular face clamping mechanisms, wherein the upper portion of the base is fixedly provided with a base platform, and the position, corresponding to the lower side of the base platform, of the base is provided with a base cavity; the die joint face limiting base is arranged on the upward side of the base platform and is located in the middle of the length direction of the base platform; the die outer circular face clamping mechanisms are arranged on the upward side of the base platform, one of the die outer circular face clamping mechanisms corresponds to the left side of the die joint face limiting base and the other die outer circular face clamping mechanism corresponds to the right side of the die joint face limiting base. The clamping requirement of a numerical control machining center machining a reference position on the end face of a semi-die blank of the die can be met; due to the fact that the overall structure is simple, the clamp is very convenient to manufacture and assemble, a good automatic clamping effect can be reflected, and the clamping operation difficulty of workers can be reduced.