In order to reveal the mechanism of offset effect in transition piers of continuous girder bridges in seasonally frozen soil areas under complex temperature fields, a method combining engineering field measurements and numerical simulation was adopted to establish a solid finite element model that thermal nonlinear interface at the soil-concrete interface and contact nonlinearity is considered. The validity of the model was verified through dual verification using on-site monitoring data and analytical solutions for thermal loading (curve < 5%). Based on this, a quantitative analysis is conducted on the influence patterns of sun-shade surface temperature difference effects, daily temperature difference effects, and the coupling effect of "soil lateral displacement+temperature" on the offset and stress of transition piers. The results indicate that the sun-shade surface temperature difference effect is the primary thermal factor leading to pier top offset, and its impact is significantly greater than that of the daily temperature difference effect. As the sun-shade surface temperature difference increases from 0 to 20℃, the pier top displacement, concrete stress at the pier base, and steel stress increase to 3.38 times, 12.77 times, and 2.45 times, respectively, compared to when temperature effects are not considered. Under the multi-field coupling condition of "soil lateral displacement + temperature", the contribution rate of environmental temperature to the total offset reaches approximately 96.58%, and the cumulative stress value under pure temperature action exceeds the structural accumulation value. In addition, local thermal stress concentration caused by thermal mismatch due to sun-shade temperature differences is the trigger for cracks appearing at the pier top concrete. The findings of this study clarify the quantitative weight of various thermal factors leading to bridge pier offset effects, providing a theoretical basis for setting thresholds in bridge health monitoring systems and formulating preventive maintenance strategies for bridge joint diseases.