Urea has been used as an additive for steam drive because its decomposition reaction will increase sweep efficiency and reduce interfacial tension. But its in situ re-energization mechanism has never been discussed in the literature. In this study, the physical experiment and numerical simulation were applied to investigate the in situ energization mechanism of urea in the assistance of steam drive for a superficial heavy oil reservoir. First the PVT experiment showed that the decomposition reaction of urea can increase system pressure by generating carbon dioxide and this phenomenon can become more dramatic with higher temperature. The dissolution of carbon dioxide in crude oil can reduce the re-energization effect. The core flooding experiment results showed that when the steam drive has reached economic limitation, the injected urea solution can flow through the gas breakthrough channel formed by the steam drive and generate carbon dioxide at high temperature. The increased high pressure can push injected steam into the deep area of the core and thus increase the sweep efficiency. Eventually the water cut dropped significantly and the oil displacement efficiency rose by 10%-20%. Finally, a field-scale mechanism model was built using CMG-STARS software to investigate the performance of urea. The results showed that at the reservoir condition, the urea still exhibited excellent re-energization effect for a superficial reservoir. But due to the high density difference, the main contribution to enhanced recovery was that the increased pressure caused higher sweep efficiency of steam-carbon dioxide combination in upper layers. Also with increment of formation pressure, the re-energization effect will become less.