A portable detecting device based on image measurement is developed to study wheel-rail geometrical contact state and make it visible. Two high speed CCD cameras are installed on both side of the rail to capture wheel-rail contact images synchronously while wheels passing through the inspecting area in high speed. Wheel-rail profiles are drawn out by specially designed image processing arithmetic, abnormal geometrical contact states of wheel-rail are recognized, and wheel-rail contacting track along inspecting area are worked out. This paper introduces inspection principle and emphasizes on parameters designing of high speed cameras. Installation feasibility and inspection capability are validated by many experiments, such as static test, dynamic tests in low speed and high speed.