Abstract: This paper presents a technology to address the deficiency of conventional three-in-one drilling technology for offshore oilfield in release of surface casing. First, the conventional borehole and casing sizes are optimized according to the API standards – the surface borehole size is changed from Ø406.4 mm to Ø368.3 mm. Second, the integrated three-in-one wellhead system is optimized to a split three-in-one wellhead system, which allows for the construction of three wells independently. Third, the occupying drilling tool is optimized by changing it designed size to Ø368.3 mm, enabling the subsequent passing of Ø355.6 mm bottom hole assembly (BHA). The occupying block is designed with an effective length of 350 mm. The precision length-matching method is adopted to ensure that the occupying blocks of two occupying drilling tool strings coincide deeply. The flow groove on the occupying block is rectangular, in order to avoid the inter-embedding of adjacent occupying blocks. Finally, the borehole trajectory is designed properly depending on the well location to achieve a rapid separation of borehole. By virtue of this technology, three wells are operated in the Ø914.4 mm conductors in one slot, while the surface casing is effectively separated. Field applications show that such measures as optimization of directional borehole trajectory, optimization of BHA, optimization of drilling fluid performance, adjustment of drilling parameters, maintenance of casing in lifting state, and optimization of casing string can help effectively improve the shallow build-up rate, ensure the sidewall approaching of surface casing, and keep the casing to be run successfully and separately effectively during three-in-one drilling, which are greatly significant for economic and efficient development of offshore oil and gas fields.