Abstract: To describe the complex physical properties of biological tissue,the interaction model among small balls is proposed.There are two kinds of interactions in the model:spring-damping and ballcontaction.The physical model of the two kinds of interactions is presented and the position variations of small balls are calculated.Taking gallbladder for example,the deformation and cutting simulation with the external force in the virtual reality environment are realized.The simulation results show that the model can effectively reflect the viscoelasticity and incompressibility of soft tissue,and can veritably reappear the biomechanical properties of soft tissues in virtual surgery.