A special fixture for the nut carving was designed, by adding a spring, the elongation of a thimble could be automatically adjusted according to the shape of nut,so clamping was reliable and loading and unloading were convenient; two clamp heads were made of rubber materials and there was a small elliptical holes at front end, then the existing questions that common fixture could not accurate positioning, easy to destroy nut due to nut irregular shape and fragile were solved.The effect of quick self centering and clamping nut automatic is realized.