With the increasing integration of renewable energies, power imbalance has become a critical issue in developing smart grids. In order to maintain the balance between power supply and demand, extra generation capacities of power plants are usually reserved by the grid at the supply side. The possibility of providing frequency controlled reserve at power demand side (i.e., industrial and residential sectors) has been approved in previous studies. However, the alteration potential and the control strategy of large-scale power demand system (e.g., air-conditioning system) in commercial buildings has seldom been studied. This paper therefore investigates the cooling demand alternation potential of commercial buildings and estimates the power demand reduction potential of the chiller(s) aiming to provide effective power information for grid operation. On the premise of indoor thermal comfort, a chiller demand limiting strategy is developed based on building thermal models. Compared with conventional indoor temperature set-point reset strategy, the developed strategy can provide an accurate estimation of power demand reduction in advance, and enable a fast response fulfilling the operation requirements of the grid. Simulation tests have been conducted to estimate the potential of reserve and investigate the impact on the thermal comfort when adopting the developed strategy.