With the widespread application of optical lenses in industrial automation inspection，it is of significant importance to evaluate the imaging quality of lenses quickly and accurately. To meet this demand，modulation transfer function（MTF）measurement instruments have been developed and put into use. The primary measurement methods include the vertical slit method and the slanted edge method. However，the vertical slit method requires strict alignment of the slit with the sensor，which is difficult to achieve in practice. On the other hand，the slanted edge method involves differentiation during the measurement process，leading to unavoidable noise amplification，which affects the measurement results. To address these challenges，an improved inclined slit method is proposed in this paper. This method overcomes the noise amplification problem associated with the slanted edge method and is suitable for slanted slits，thereby enabling precise MTF measurement. The proposed method uses multi-frame image pixel averaging to remove random noise and obtains the slit image. It extracts the pure background region of the image to compute the pixel mean as the background noise value，and then subtracts this value from the image to remove background noise. Additionally，a new centroid calculation method is introduced for the slit image. First，the highest pixel value in a line or column of the line spread function（LSF）in the image is used as the center，and pixels to the left and right are traversed to determine the effective slit region. Data from this region is then processed using a cubic spline interpolation algorithm to obtain a sub-pixel level centroid position. With the centroid position of the current row or column as the center，discrete sampling of pixel positions is performed at 0.25 times the camera pixel size sub-pixel intervals and categorized into bins. The mean pixel values along the slit direction within the same bin are calculated to obtain a fourfold oversampled sub-pixel level line spread function. The improved slanted slit method not only accurately calculates the slant angle of the slit but also precisely measures the lens MTF. A detailed analysis of factors affecting MTF measurement accuracy is conducted，and the results show that，compared with advanced foreign instruments，the measurement error of MTF values is less than 1% within the Nyquist frequency before oversampling. After oversampling，the measurement error is less than 1.3% within 2.6 times the Nyquist frequency. The experimental results validate the effectiveness and practicality of the proposed method，demonstrating that it meets the needs of industrial production. This method effectively addresses the issues of high cost and limited control associated with purchasing advanced MTF testing instruments from abroad.