Abstract：Fire system, which is composed of fire and its effect factors, is a complex system. In this paper, the recent studies on fire complexity are introduced. The frequency-size distribution of forest fire satisfies a power law relation that is invariant with time and system size. Actual forest fire data accords well with the simulation and calculation results of the forest-fire model, indicating that the forest fire system is a self-organized critical system. If more effect factors such as precipitation and forest scale are considered, the forest-fire model gets better accordance with actual forest fire data. The city fire data is also found satisfying power-law relation invariant with system size and time. A steady power-law relation has practical implications. The future fire distribution can be forecasted by enough historical fire data, the occurrence probabilities of large fires can be calculated by those of small fires, and the forest-fire model can be used to assess the long-term performance of fire protection policies.