The Ring Oscillator Physical Unclonable Function (RO PUF) is an efficient and safe identification and authentication security primitive. However, since the RO oscillation frequency may be related to the type of FPGA used, its safety is difficult to guarantee. Based on the Spartan-3E S500 of Xilinx, this article summarizes the characteristics of the RO frequency distribution in the RO array and proposes a frequency compensation strategy. Combining the two existing comparison strategies, the safety of RO PUF before and after using the frequency compensation strategy is evaluated from the two perspectives of the average bit average probability of being successfully predicted and the information entropy density. Experimental results show that the frequency compensation strategy can greatly improve the safety of RO PUF.