Riserless drilling is usually used in ocean drilling where downhole incidents often happens in loose and brokenformations due to instability of the hole wall. The most ideal way to protect the wall is to seal the unstable formation with casing. With the traditional casing RIH method, the procedure is complicated with long operation time, susceptible borehole shrinkage and collapse, difficulty in running the casing to the predetermined position, high construction risk and high cost, etc. This article introduces a casing-while drilling technique by which drilling and casing is combined into one step through a complete set of equipment such as the underreamer, mud motor, and casing running tool. The technique simplifies the operation procedures, saves operation time significantly, and has solved the technical difficulties in casing operation in deep sea riserless drilling in complex formation, such high operationrisk, high cost, etc., providing technical support for deep sea core drilling.