Aiming at the problem of large error in manual inspection of welding gap of Y-type pipe joint set, a visual inspection method for welding gap of Y-type pipe joint set based on improved Canny algorithm was proposed. Firstly, the Y-joint clearance image is enhanced with multi-scale details to reduce the influence of image details missing caused by filtering. Secondly, Otsu maximum inter-class variance method is used for threshold selection, which reduces the defects caused by manual selection of threshold. Finally, an algorithm is designed to extract the characteristic points and calculate the clearance, which can measure the clearance of Y-type pipe joint. The experimental results show that the method used in this paper can accurately and directly detect the clearance amount of Y-type pipe joint pairs, which meets the requirements of joint clearance quality detection in actual production.