Abstract: To reveal the spatial distribution characteristics of main-cable thermal deformation in long-span suspension bridges and to improve the reliability of thermal-response extraction at asymmetric monitoring sections, this study establishes an analytical model of main-cable thermal displacement based on the structural health monitoring system of a 1688 m-class anchored suspension bridge and validates it using long-term measurements. To reduce the interference of daytime temperature gradients and random traffic loads, quasi-static data collected before sunrise from 4:00 to 6:00 were selected, and a linear mapping between vertical displacement and effective temperature was obtained with a coefficient of determination of 0.93. Based on the parabolic configuration of the main cable, the theoretical vertical-displacement ratio between the quarter-span and mid-span sections is derived as 0.75; the measured ratio is 0.73, which is close to the theoretical value. Although the measured thermal sensitivity is about 10% lower than the analytical prediction, the equivalent reduction coefficients at the two sections are similar, indicating that tower, stiffening-girder and hanger constraints mainly reduce the absolute displacement amplitude, whereas the normalized spatial distribution is still governed primarily by the cable geometry under the examined conditions. A projection analysis further shows that, for a 50 °C temperature-difference case, the first-order linear projection error caused by longitudinal thermal movement is approximately 0.04%, and therefore has little influence on vertical-displacement extraction. The results provide a reference for thermal-displacement estimation, asymmetric-section shape monitoring and environmental-effect separation in similar long-span anchored suspension bridges.