Abstract: In order to effectively utilize limited offshore slots and explore the remaining oil and gas in Wenchang old oilfield in the western South China Sea, a short-radius horizontal well with controlled-trajectory of casing window sidetracking is proposed to achieve directional potential exploration and maximize the single well oil drainage area. In response to the current technical difficulties of real-time trajectory control of short radius wells, PDM (positive down-hole mud-motor), flexible titanium alloy drilling pipes, flexible while drilling inclinometers, and specially designed PDC bits are successfully developed, which are able to meet the requirements of 10 times faster buildup rates compared with conventional drilling tools and a horizontal section exceeding 200 m. During application process, Well WC-X1 was sidetracked from the ϕ 244.48 mm casing and drilled 127 m, increasing the inclination from 0.56° to 88°. Subsequently, based on the real-time inclination and gamma data uploaded by the flexible drilling measurement tool, the formation data was determined in real time, achieving precisely trajectory control to complete a 220 m horizontal section in the sand body. The oil recovery rate in the oil formation increased by 46.5% compared with the old well, and the accuracy of trajectory control reached ±1.5°. This related technology provides new ideas for the development of oil and gas in the bottom layer in offshore old oilfields.