Abstract: Ultra-short radius horizontal wells play an important role in the stimulation of old oil and gas wells and the potential tapping of remaining oil. The commonly used ultra-short radius horizontal drilling technology with flexible bottom hole assembly (BHA), however, faces the problems of long tripping periods and high operation costs. In contrast, coiled tubing drilling tools feature high drilling efficiency with low costs and are safe and reliable. Hence, this study combined the advantages of coiled tubing and flexible BHA and developed key tools for ultra-short radius horizontal drilling, including large-torque positive displacement motors (PDM) at high and low rotary speed, optimized flexible BHA, stacking whipstock and build-up PDC bit, etc. Moreover, supporting operation processes were optimized. As a result, the ultra-short radius horizontal drilling technology combining coiled tubing and flexible BHA was preliminarily developed. The technology was first applied in a field test in Well Ling 72-5CZ of Jianghan Oilfield, and an ultra-short radius horizontal lateral well of 30.21 m was drilled, with an average rate of penetration of 0.8 m/h. The trajectory was in good agreement with the designed one, and the horizontal section has an average inclination angle of 88.17°. The field test results verify that the ultra-short radius horizontal drilling technology combining coiled tubing and flexible BHA is feasible, and it can provide a new technology for the potential tapping of remaining oil and efficient development of low-permeability oil fields.