Early warning is an important function of the emergency response system for the urban power grids. It evaluates the risks of disaster events and releases the warning information accordingly. In this work， a novel method for the early warning is proposed. The method is based on the power failure risk analysis of the distribution system. By setting up the probability model of the devices'outages， the power failure risks can be evaluated for the partial or global urban power grid. These risks are used to define the corresponding warning levels of the disaster events. Given that the emergency controls are considered， the power failure risks obtained are also helpful for optimizing the prevention plans against the impacts on system security and reliability from disasters. A test case of the standard IEEE system is presented to illustrate the capability and feasibility of the proposed early warning method.