Abstract: This paper analyzed frequency characteristics of power systems with high penetration of renewable resources under power steps, and proposed a practical method for quantifying the characteristics of system common mode frequency (i.e., common mode component of frequency response). Firstly, an approximate method using a transfer function model with a unified structure to describe the frequency-active power response of devices was proposed, and on this basis, a simplified transfer function model describing the system common mode frequency response characteristics was formed. Secondly, based on the simplified transfer function of the system, analytical expressions describing the common mode frequency nadir and the average rate of change of frequency (RoCoF) were obtained, and two simple metrics, "drop depth coefficient" and "drop slope coefficient" were proposed to quantify frequency characteristics. Finally, simulations were used to verify the effectiveness of the proposed frequency characteristic quantification method.