Modulation Transfer Function (MTF) is a key parameter for evaluating the imaging quality of optical systems, and its measurement accuracy directly affects the reliability of optical systems performance evaluation. In the MTF measurement technology based on the slit method, the systematic error introduced by the slit width is a critical factor constraining the measurement accuracy. To address this issue, this study proposes an error correction method based on a deconvolution algorithm. By constructing a complete measurement model that incorporates the actual convolution effect of the slit, an exact analytical expression for the Line Spread Function (LSF) is derived using the Struve function, enabling both theoretical modeling and experimental analysis of the systematic error. In the experiment, a standard 1m-wide slit is used to build a high-precision measurement system, and the MTF of three typical microscope objectives is measured and verified. The results show that in the two important spatial frequency ranges of 20-55 lp/mm and 70-100 lp/mm, the absolute error of the corrected MTF measurement remains below 0.02, confirming the accuracy and reliability of the MTF measurement. This study provides an effective solution for precise MTF measurement of optical systems with high numerical aperture and low aberration.