The characteristics of power transmission of vibration isolating system with passive mounts on a flexible plate foundation was studied. The theoretical model considering two inclined mounts was established, and the power transfer function of the system was deduced using the mechanical admittance method. In numerical simulations, infinite and finite plates with both thick and thin dimension were all taken into account. The characteristics of power transmission of the system was investigated through numerical simulations, and some conclusions that can benefit the practical engineering were obtained.