This paper presents an approach to animate realistic interactions between tree branches and raindrops, based on physical theory. Tree branches and petioles are represented by a model namely ETPSM (extended three-prism spring model) with three-prism structures, which can be flexibly controlled by four kinds of spring systems. The branches and leaves are animated due to the bidirectional transference of kinetic energy between raindrops and the branch system. The interactions between them can be well simulated by an efficient technique, specially designed for liquid motion on non-rigid objects with hydrophilic surfaces. When the branches are impacted by a raindrop, they will vibrate and twist, and the raindrops will flow along the leaf vein, merge into larger ones, or hang on the leaf apex, and the branch will rebound and vibrate for a while after the raindrop falls off the leaf, finally into a rest place. Experimental results show that this approach can be used to efficiently and realistically simulate these interactions between tree branches and raindrops; meanwhile, it can efficiently and easily simulate the elastic deformation of rod under the influence of external forces, such as rotation and vibration.