This paper proposes a novel approach to animation realistic interactions between tree branches and raindrops in a physically-based way. A new elastic model using a three-prism structures is presented to flexibly bend and twist tree branches naturally in the first time. Various distinct forms of interactions when or after raindrops hitting on tree branches can be well simulated using a new efficient technique specially designed for liquid motion on non-rigid objects with hydrophilic surfaces. Experimental results indicate that our approach can be used to simulate the interactions between tree branches and raindrops efficiently and realistically.