This paper addresses the development of a dynamics simulator that for a biped robot on loose soil. Especially, the paper focuses on the method of sinkage calculation which is one of the characteristics for walking on loose soil. The calculation algorithm is changed dynamically according to the support condition of the legs. Considering ground projection of center of mass (GCoM) as a center of rotation of a robot, angle of trim is also estimated. Additionally, to describe the dynamical variation of sinkage, a filtering method is proposed for the calculation result. By applying the proposed method, sinkage phenomenon of a biped robot during stepping is simulated realistically.