The invention discloses a decentralized and hierarchical control method for emergency microgrid composed of portable emergency power. The decentralized and hierarchical control method comprises the steps of carrying out a regional division on city power distribution and utilizing the portable emergency power to compose the emergency microgrid for providing an emergency service for an important load in certain region, after large area power cut caused by disasters; acquiring a target frequency of emergency microgrid through the decentralized and hierarchical control of the frequency and then utilizing a speed controller to adjust rotate speed of a turbine according to the target frequency and changing active power outputted by the potable emergency power and finishing the adjustment system frequency; acquiring a target power through the decentralized and hierarchical control of the power and utilizing an excitation system to adjust an exciting voltage according to the target power and then keeping the generator terminal power of the portable emergency power within a permissible range. The decentralized and hierarchical control method improves the emergency service capacity after large area power cut due to urban power grid suffers extreme disasters.