Abstract: A simple scheme with low cost and high precision to measure the straightness of deep holes' generatrices was proposed. Based on the principle of EST, the surface of the hole was adopted as the base of measurement, instead of the guideways which are generally used. At the same time, with the example of three point method, the mathematical expression of the measurement uncertainty was deducted, and a simplified way to judge its feasibility was given. The presented scheme can effectively solve the problem of generatrix straightness measurement for deep holes.