Abstract: A visual-based experimental method is proposed for the quantitative evaluation of anti-loosening performance in bolts of different materials by correlating measured rotation angles with preload loss. Firstly, the conventional visual angle measurement technique was enhanced via an improved color segmentation strategy and the addition of a perspective-correction module. Secondly, a standardized line-marking tool was developed using 3D printing to improve the precision of rotation-angle measurement. Subsequently, bolts were deliberately loosened to reduce preload in equal increments, and the resulting rotation angles were recorded. The relationship between rotation angle and preload loss was then fitted using an exponential-decay model, leading to the design of an experimental protocol and the definition of a composite anti-loosening performance index. Finally, the method was validated on multiple bolts of identical dimensions made from various materials. Experimental results demonstrate that this approach can effectively quantify bolt anti-loosening performance at the research level by linking rotation angle to preload loss, and it shows strong potential for practical application.