The continuous cyclical load disturbances can lead to forced power oscillations of a power system. Based on the single-machine infinite-bus system and the two-machine system analysis， the mechanism of the forced power oscillation caused by cyclical load disturbances is presented. Then， the influence factors of the load power disturbance on the system inherent oscillation modes are obtained using modal analysis of multi-machine systems. The key factors that influence the oscillation are analyzed. Finally， the simulations of a 4-machine 2-area system and the New England 10-machine 39-bus system are carried out， which contribute to a better understanding of the forced power oscillations caused by continuous cyclical load disturbances.