This paper aims to investigate the wellbore thermal behavior in multi-gradient drilling (MGD). Based on the principle of energy conservation, a synthetic transient heat transfer model was established, in which the mass and heat transfer of lightweight glass microspheres (GMSs) were considered for the wellbore fluid region, and the dynamic variation in the moving boundary was considered for the drill bit and separator region. The synthetic model was solved using the combination of finite volume method and dynamic laying method. The accuracy and capability of the model were verified using field measurement data and previous typical models. The calculation results suggested that the annular temperature near the separator dropped suddenly due to the mass and heat transfer of GMSs. Additionally, the dynamic drilling of the drill bit would increase the bottomhole temperature continuously. Furthermore, a sensitivity analysis indicated that when the GMS concentration increased from 5% to 45%, the annulus temperature at the separator reduced by 9.15%; when the distance between the drill bit and separator increased from 1100 m to 3000 m, the bottomhole temperature increases by 3.99%; and the effect of the separator number on annulus temperature combined the dual role of separator position and GMS concentration. (C) 2019 Elsevier Ltd. All rights reserved.