There are some of multiple-sensor methods used for on-line measurement of the straightness of precision machine tool and machined workpiece， But，all of these methods are only used for long workpiece measurement. The OEST method proposed in this paper is a new one that can be used for short workpiece measurement. 1t is based on the analysis of frequency domain and optimum search in time domain. It can overcome the mismatch problem in boundary area caused by shift in frequency domain. And，computer simulation and experiment results prove that this method is easy and efficient.