Ultra high frequency (UHF) radio frequency identification (RFID) systems have many advantages, such as low tag cost and farther identification range. Therefore, it is meaningful in the field of intelligent transportation such as electronic toll collection (ETC), etc. However, it is difficult to accurately estimate the tag identification area in practical applications for the random multipath propagation of radio waves. For ETC scenarios often have in common geometric features. This paper firstly analyzes the constitution of the tag received multipath waves, and then proposes a prediction method of tag identification area. The method is based on geometric optics theory and takes the scenario geometric features and antenna RF characteristics into consideration. Actual tests and simulations showed that the method can accurately predict the tag received power and identification area. The prediction accuracy is higher than the empirical models, and the computational complexity is lower than the electromagnetic computation software.