Partially earth-anchored cable stayed bridge can be constructed using the free cantilever method (FCM) which is the most widely used technique for bridge construction. However, it is difficult to apply the set back & reset back work, because of the existence of the earth-anchored cables. Due to such difficulty, an innovative key-segment closure technique was proposed using the thermal prestressing method (TPSM). And solar radiation causes unequal temperature distribution on the steel box girder section. Thus the unequal temperature distribution is a critical factor for detailed construction management. So this study aims to propose a prediction method for temperature variation and examined the construction details of key-segment closure. The proposed method considers solar exposure and wind velocity during the prediction of temperature variation, which is the most momentous parameters for the temperature distribution in the steel-box girder. In order to reach target temperature of the steel box girder, the analyses are performed considering seasonal effects with different target temperatures. The results showed that the proposed method released accurate results and maintain the target temperature consistently. It can be concluded that the proposed method can be effectively used in predicting heating temperature, time and the time required of heating during a closure work in cable-stayed bridges. However, it is necessary to have solar exposure and wind velocity data in order to get accurate result.