Linewidth is one of the critical indicators affecting the performance of microstructured devices. With the continuous development of the micro-nano processing level, the current microstructure is becoming more and more refined, and the measurement precision requirements of its geometric critical dimensions are also increased. For example, a line or groove structure's critical geometric dimensions mainly include depth, linewidth, and sidewall angle. The linewidth described in this paper is the minimum geometric feature size at the top of the trench structure with a step edge. This study focuses on the measurement method of line width, whose scale covers the range of several microns to tens of microns. Optical microscopy is the mainstream method for fast and non-destructive measurement of microstructure linewidth, but the imaging diffraction limits the measurement precision. This paper analyses the edge positioning error that limits the measurement precision starting from the microscopic imaging's theoretical model of linewidth measurement. The image point of the sample step edge is superimposed by its own diffraction spread and the diffraction spread of nearby points, which affects the positioning of the step edge point. Suppose the light intensity function of the image point affected by the diffraction effect on the step edge can be differentiated; in that case, the influence of other points near the step can be removed, and the light intensity response of the step edge point or a very small range on the image surface can be enhanced, so that the precise positioning of the edge can be achieved. Therefore, this paper explores the sensitive detection principle of light intensity differential induced by step edge diffraction to achieve high-precision detection of groove linewidth. On this basis, an attempt is made to propose a linewidth microscopic measurement method based on translation difference. A high-precision piezoelectric ceramic micro-displacement platform is added to the traditional microscopic imaging method, the sample is fixed on the displacement platform and displaced along the line width direction, and two micrographs before and after translation are collected in one step. Subtract the micrographs to obtain a differential image, and use the differential pulse to solve the problem of step edge localization. Two differential images are obtained by two-step translation, the relationship between the differential and the displacement is established, and the sub-pixels are calibrated with high-precision displacement. Therefore, we can take advantage of the high resolution of differential pulse positioning to improve linewidth measurement precision. Besides, the platform's high resolution ensures the measurement results' accuracy by calibrating the sub-pixels with micro-displacement. In particular, we customize a standard groove template with a linewidth of 30.00 μm as the test sample. The average value of the measurement is 30.03 μm, and the standard deviation is 0.005 μm after ten measurements. The uncertainty of this method has been analyzed, and the final synthesis Uncertainty is 0.37% (k=1). The results show that with the help of the nano-precision micro-displacement mechanism and the diffracted light intensity differentiation of the groove step edge, the limit of the optical diffraction resolution of the microscope can be broken, and the accuracy of the non-destructive detection of the groove line width by the direct imaging method of the optical microscope can be significantly improved. On the other hand, although this paper takes a standard sample with a single groove as an example to verify the feasibility of the method, from the measurement principle and layout, as long as the numerical aperture of the microscope objective is sufficient to resolve the groove, the translation difference method can measure the linewidth of all grooves in the field of view of the microscope objective at one time and has high detection efficiency.