Abstract: Well TK430H is the first testing under-balanced well drilled in the Ordovician system of Tahe oilfield, with a designed depth of 6090m, and a real measured drilling depth of 5780m. It is the deepest under-balanced well drilled in China to date, as it has a TVD of 5420.11m, a horizontal displacement of 484.51m, hole inclination of 89.0°, and an azimuth of 251.56°. The under-balanced drilling plan and its field operation is described. This well realized drilling while lost, and it was successfully drilled to bottom, which showed the feasibility of applying under-balanced drilling technology in Tahe oilfield, and provided a new alternative of developing this oilfield.