Modern power systems utilize information, communication, and computation technologies for control and coordination of the various subsystems and enhancement of the system's operating performance. In this paper, we develop a model to simulate cascading failure in a cyber-coupled power system considering the system-level control provided by the cyber network. In the events of failure of components or subsystems, the states of the power network will be monitored, collected, and transmitted in real time to the control center, where control algorithms are performed to generate commands for controlling the power at each node. We develop an optimization algorithm for the control center that maximizes power generation and avoids power imbalance between generators and loads and overloading on each component. The vulnerability of the cyber-coupled power system to initial component failures is assessed based on the model and cyber faults that damage the state-monitoring and controlling function of the cyber layer are considered. Simulation results demonstrate that a power system coupled with a cyber control system can effectively reduce the cascading failure risk. However, the control dysfunction of the cyber layer leads to catastrophic cascading failure and intensifies the extent of power outage, making the system more vulnerable to cascading failure.