With wireless devices, the specic absorption rate (SAR), which quanties the human exposure by electromagnetic waves, is usually measured by a traditional measurement system where a probe measuring amplitude is intensively moved inside the phantom so that the peak spatial-averaged SAR is accurately estimated. However, such measurement process is time consuming considering varied congurations (device position, working mode, etc.) for the full-compliance test and massive production (e.g., mobile phones). Types of fast measurement systems have been developed to overcome this diculty but estimation discrepancies are observed for dierent systems. Investigations are made on the estimation discrepancies by simulating the fast measurement system based on techniques of eld reconstruction and comparing results from the traditional measurement system and the fast ones.