A double-working-medium expander used for a two-stage organic Rankine cycle, comprising a cylinder body, a rotor disposed inside the cylinder body and provided with a plurality of slip sheets in a radial direction of the cylinder body, and a rotary shaft fixedly connected to the center of the rotor, wherein, the cylinder body is of an annular structure formed by two semi-oval structures which are in butt joint with unequal semi-major axes and equal semi-minor axes; the outer peripheral surface of the rotor can be rotationally tangent to the inner peripheral surface of the cylinder body at the butt joint position of the two semi-oval structures of the cylinder body; a low-temperature cycle volume for expanding the low-temperature working medium with a large flow and a small expansion ratio and a high-temperature cycle volume for expanding the high-temperature working medium with a small flow and a large expansion ratio are formed among two sides of the rotor and the cylinder body, respectively; and, a first fluid inlet and a first fluid outlet, which are arranged on the wall of the cylinder body, are communicated with the low-temperature cycle volume, and a second fluid inlet and a second fluid outlet, which are arranged on the wall of the cylinder body, are both communicated with the high-temperature cycle volume. In the present invention, two working mediums in the two-stage organic Rankine cycle can do expansion work in an expansion mechanism.