With the rapid development of modern mechanical processing industry, its requirements for workpiece size accuracy are gradually increasing. However, there are still certain deficiencies in the dimensional accuracy of workpieces in the current mechanical processing process, and the methods for detecting workpiece size deviation are often affected by image noise and algorithm defects, leading to a decrease in the accuracy of workpiece size deviation detection. Therefore, a research on online detection method for size deviation of machined workpieces based on laser sensors is proposed. Select and configure laser sensors as the basis to obtain laser data for measuring the size of machined workpieces. Use the amplitude limiting filtering method to remove noise data from the laser data for measuring the size of workpieces, calculate the measured dimensions (length, width, and height) of machined workpieces, load the expected size data of workpieces, and measure the difference between the two. Based on this, determine whether there is a deviation in the size of machined workpieces, Thus achieving online detection of workpiece size deviation. The experimental data shows that the deviation detection results of the mechanical processing workpiece size (length, width, and height) obtained after the application of the proposed method are the same as the actual deviation, with a maximum error of no more than 0.5 mm and a distinguishable minimum deviation of 0.2 mm, fully confirming the effectiveness of the proposed method.