A method and device for informing user address are disclosed. The method comprises: a defined network allocating, according to a user address prefix provided by a mobile network, a user address to a mobile network user accessing the mobile network from the defined network; and the defined network informing the mobile network of the allocated user address. The disclosure address the problems of complex flow for allocating user address and low allocation efficiency, and how to inform a mobile network of an allocated user address existing in the related art, thus achieving the allocation of an user address and improving informing efficiency.