With 6-h and daily reanalysis data from NCEP/NCAR, we studied the mechanisms for the sudden Arctic warming on 29 December 2015. The signal area average method and heat flow equation for temperature estimation were used in this study. It revealed that there was a sudden warming in the Arctic on 29 December 2015 and the maximum temperature increase was over 25℃ or more. There existed strong southerly winds with speeds larger than 20 m s^-1 over the warming area during the same period, which was driven by the combined effects of a strong cyclone and anti-cyclone. The strong moist southerly winds brought warm temperature to the Arctic region and enhanced cloud cover over the sudden warming area. The temperature advection turned from cold pattern to warm pattern from 28 December to 29 December, which corresponded to the rapid increase in temperature and indicated that warm temperature advection played a vital role in the sudden Arctic warming. The enhanced cloud cover led to deceases in the upward long-wave radiation flux in the dark night, contributing to the temperature rise. For the sudden warming, the contributions of temperature advection, vertical term, and non-adiabatic effect were 20.4%, -9.9%, and 89.5%, respectively. Therefore, the temperature advection and non-adiabatic effect were responsible for the sudden Arctic warming, and the non-adiabatic effect was a more important factor.