As the scale of the modern power system gradually increases, high integration of renewable energy sources and diverse types of loads have become dramatic features of power systems, significantly affecting the stability of the system frequency response. It becomes increasingly important to establish a more accurate system frequency response (SFR) model to analyze system frequency responses and guarantee system frequency stability. However, existing SFR models, to some extent, neglect the voltage and frequency dependency effect of system loads, creating errors in SFR models. This paper proposes an improved SFR model that considers the aforementioned load effect. The model rectifies the load contributions to total system disturbance based on various types and proportions of constant impedance loads and constant power loads. Simulation results validate that the improved SFR model can significantly reduce the analysis errors for the system frequency dynamics compared to conventional SFR models.