Abstract: Firstly, the ORC organic Rankine cycle technology and the structure of the scroll expander are briefly described. Then, the geometric models of the constant cross-section (circular involute) scroll expander and the variable cross-section (circular involute-higher curve-circular involute) scroll expander are created according to the scroll profile meshing theory; and the corresponding leakage model is established. Finally, change trend of the volume of each working cavity and the length of the radial leakage line is analyzed, which lays a foundation for selecting the line structure of the scroll expander.