After RFID technology implemented, the first face of the problem is how to maintain the stability of the high tag read rate, but the applications in the environmental interference, the electronic tag attached to the position, the electronic tag antenna design and performance, the reader antenna type and performance, reading gate set point, the antenna set up position and angle, product packaging and its contents the same pallet of materials and the total number of electronic tags and many other factors, all affect the read rate of RFID tags. Similarly, in the the the National Freeway ETC import RFID UHF technology, the read area of the electromagnetic field intensity adjustment operation is a difficult challenge. Because the National Highway Traffic characteristics of high traffic volume per unit time, and the system is not available offline (to stop toll collection) and stop vehicle access to the field strength measurement and adjustment of the read area. If can be make a installation SOP of the reader antenna’s pitch adjustment, will be able to eliminate the poor reading rate. It will be greatly improve reading rate of the portal. This study is to explore how to use the minimum turn on power of RFID tag to fine-tune the electromagnetic field strength distribution of the portal. To propose used HFSS finite element analysis software to create a portal model. To simulate the portal’s strength of the electromagnetic field distribution through the tool that simulating cargo height and width to fine-tune the electromagnetic field strength distribution of the portal. And via check the portal’s electromagnetic field strength distribution by RF spectrum analyzer. Finally, by reading rate tests of the Wal-mart receiving/shipping portal and ETC(electronic toll collection) portal to verify the proposed research method. The proposed research method proves the relationship between the electromagnetic field strength and the electronic tag read rate.