This paper formulates a dynamic economic load dispatch (ELD) problem while considering the demand response (DR) in smart grid. To solve the novel ELD problem, an equivalent SWMP is presented first, and then a distributed Alternating Direction Method of Multipliers (ADMM) algorithm utilizing the average consensus protocol is proposed. In particular, the impact of the transmission power losses is also discussed. A sufficient condition that can ensure the convergence of the distributed algorithm is derived along with theoretical analysis. The distributed ADMM algorithm maximizes the social welfare and guarantees the instantaneous power balance. Case studies on the IEEE-39 bus system demonstrate the performance of the proposed algorithm.