This study exams the main sources of inter-model spread in Arctic amplification of surface warming simulated in the abrupt-4×CO2 experiments of 18 CMIP6 models. It is found that the same seasonal energy transfer mechanism, namely that the part of extra solar energy absorbed by Arctic Ocean in summer due to sea-ice melting is temporally stored in ocean in summer and is released in cold months, is responsible for the Arctic amplification in each of the 18 simulations. The models with more (less) ice melting and heat storing in the ocean in summer have the stronger (weaker) ocean heat release in cold season. Associated with more (less) heat release in cold months are more (less) clouds, stronger (weaker) poleward heat transport, and stronger (weaker) upward surface sensible and latent heat fluxes. This explains why the Arctic surface warming is strongest in the cold months and so is its inter-model spread.