The reading region of ultra high frequency radio-frequency identification (UHF RFID) system is affected by polarization mismatch, reader antenna height, coherent interference and so on. Based on the principles of RFID technology and ray-tracing method, a complete propagation model in real environments is derived. Utilizing recognition rate of target region (RRTR), a method of evalualing the performances of a system is presented. In order to obtain the maximization of RRTR, two solutions including tag diversity and phase switch are proposed, which combines the theories of electromagnetic propagation and coherent interference suppression. The values of RRTR under different parameters are measured in outdoor and indoor environments with a commodity hardware. The measurement results show that the values of RRTR can be enhanced by 10% and 7.6% with the two solutions respectively.