Future power systems will be characterized by low levels of inertia and limited frequency regulation capacity due to the widespread use of renewable energy sources. Furthermore, the response of different types of loads to system disturbances significantly affects the frequency dynamics. To address these issues, this paper proposes an aggregated system frequency response model considering load dynamics. Initially, a dynamic model of different loads is established, followed by the derivation of a small-signal load model that affects the active power imbalance of the system. The active power variations of loads are categorized into three components: load voltage dynamics, load frequency dynamics, and load inertia contribution. These components are incorporated into the system frequency response model, which accounts for load active power dynamics. The final output is an aggregated reduced-order system frequency response model, where the aggregation is primarily weighted by the primary frequency regulation capability of the load, load capacity, and rotor kinetic energy. Finally, the accuracy and effectiveness of the proposed model are validated using the WECC 9-bus test system with power electronic sources, and the influence of load parameters on the frequency stability indicators is analyzed.