In this paper, an interactive dynamic background scene generating and editing system is proposed based on improved motion graph. By analyzing the input motions with limited frame length and their metadata, our system could synthesize a large amount of various motions to yield a composing dynamic background scene with unlimited frame length by connecting the motion pieces through smooth transitions based on their motion graph layers. The motion pieces are generated by segmenting the input motions and corresponding deforming meshes spatially and temporally, while the smooth transitions connected the motion pieces are obtained by searching the best path in the motion graph layers according to the specified circumstances. Finally, the result motions are optimized by repeatedly substituting the motion sub-sequences. To design the dynamic background scene, users can interactively specify some physical constraints of the environment on the keyframes, such as wind direction or velocity of flow, or even some simple paths for characters to follow, and the system can automatically generate a continuous and natural dynamic background scene in accordance with the user-specified environment constraints.