Under-balanced drilling has been using in China for several years. UBD technology can reduce mud weight in the maximum way, and is a valuable method of minimizing formation invasion related problems. Some oilfields have found its strongpoints in exploration and development. This paper demonstrates that UB sometimes can be used to break through the barriers of HPHT, to prevent the H2S from branching out, to solve the problem of narrow margin between the formation fracture pressure and pore pressure, to find a new producing pay, and to drill through the lost circulation zone etc. So it could be a way of using UB tools or UB technology to solve the engineering difficult problem and to keep the well in safe, at the same time UB has shortage itself and will bring some troubles if it not be properly designed or controlled. Several problems in using UB tools and technology will be discussed, especially on the methods of pressure control. To enhance the reliability of UB practice, a new design choke control panel has been used in well site. This equipment has the multifunction of drilling engineering data acquisition, data transimission and storege, and especially has the function of pressure automatic control. The result of laboratory test illuminates that the function of software and data acquisition of the system work well. The result of 3 well site tests and 2560 hours work illuminates that the functions of hardware, software and the pressure auto-control of the equipment work well. It is valuable for improving UBD operation and safety. UB tools give us the approach of reducing mud density and offer the well safety in a certain extent, so we can use it in UB and NBD (near balance drilling), just for when and how much the new productivity and enhancement production has been gotten, not for when and how much the pressure difference has been remained.

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