Truss welding robot is widely used because of its long working stroke and simple structure. The straightness and parallelism of slide rails of a robot can affect the accuracy of weld and the life of the slider. Therefore, the study of the deformation law of the guide rail is very important to improve the precision of the robot. Taking the truss welding robot of large industrial gas furnace panel as an example, the deformation caused by gravity, temperature and its coupling effect was analyzed. The results show that when the robot is in a high temperature working environment, the deformation produced by the temperature is larger than that caused by gravity. In the direction of gravity, the deformation produced by gravity is opposite to that caused by temperature. In the direction perpendicular to the gravity, for the horizontal beam guide rails, the deformation caused by gravity is the same as that caused by the temperature. For the vertical beam guide rail, the directions are opposite in the normalized length range of 0~03, and the two deformation directions are the same in the normalized length range of 03~10. Both theoretical analysis and practice show that the straightness and parallelism of the guide rail of truss welding robot of the large industrial gas furnace panel are less than the allowable value of the engineering, and meet the engineering requirements. The analysis results are of great theoretical significance for the design of similar truss robots.