With the development of integrated circuits, their functionalities have been significantly enhanced. However, the increasing power consumption also leads to thermal issues. This work fast extracts an equivalent thermal model at chip boundaries in a system. Attaching the extracted thermal model to boundary surfaces of the chip, we can use fewer thermal grids to simulate chip temperatures but also include the system thermal effect. The experimental results show that the grid reduction is over 76.38% in steady-state thermal simulation and over 51.53% in transient thermal simulation.