With the massive access of renewable energy, the frequency characteristics of the system have changed greatly. In the power system where the proportion of renewable energy is high, the frequency evaluation index obtained will no longer be applicable. Therefore, this paper proposed the frequency strength evaluation index suitable for high proportion renewable energy system. Firstly, the system frequency and the unified transfer function structure (UTFS) model are established. Secondly, the error of frequency strength evaluation index caused by using the index to evaluate frequency strength in different scenarios is analyzed. Furthermore, the frequency strength evaluation index used in high proportion renewable energy system is proposed and based on the proposed index, the frequency strength evaluation steps are given. Finally, the effectiveness of the index is verified by simulation.