Horseshoe cross-section is one of commomly used sections for open-channel flow tunnel.Its geometric shape consists of four radial curves.The hydralic elements,including cross-section area,wetted perimeter,water surface width,etc.,can be expressed by piecewise functions respectively.The hy-draulic calculation of horse shoe cross-section in open-channel flows is difficult because of its complex sectional construction.The formulas of hydraulic elements and iterative formulas of critical depths for standard Ⅰ and Ⅱ type horseshoe cross-sections are derived,and how to discriminate the demarcation discharges of range of critical depths is presented.The calculated results of hydraulic elements at characteristic point are identical,therefor the continuity and correctness of the formlas at characteristic point are proved.