There is a wealth of dynamic information hidden in system responses, which is subject to the random excitations. The analytical form of electromechanical response of power system under small environmental excitation are deduced, and the existence of electromechanical oscillation characteristics in the ambient response of power system is proved from the mathematical point of view. A method for oscillation parameters identification of power system on the basis of dynamic mode decomposition (DMD) algorithm is proposed. By comparing the results of the electromechanical oscillations parameters identification with theoretical characteristic parameters and analysis of the probability statistical results show that DMD algorithm has strong adaptability, meanwhile the correctness of oscillations parameters identification by using the ambient signals is verified by the simulation analysis of IEEE 4 generators 2 area system and a regional power grid disturbance recording data.