The formation pressure system of Bohai 25 oilfield is complex. In the first stage of development, the conventional casing program in Bohai was adopted, but the frequent complex situation and long construction period led to high cost with poor overall economic benefit. Through the study on slim hole technology in Thailand bay, combined with the actual situation in Bohai, a set of optimized casing program 355.6mm hole×273.1mm casing 241.3mm hole×177.8mm casing 152.4mm hole×114.3mm liner and the support improved ROP technology was developed. Through the application tests in 2 wells, good results have been achieved with the average well depth 3607.5m and average drilling period 19.32 days, drilling cycle saved 5.37 days and drilling efficiency increased by 24% in these 2 wells than the basic design. The costs of equipments and materials used in slim hole drilling go down accordingly and better yield and significant economic benefits are received in operation with broad application prospects.