Abstract: CB30A-1 well is a direction well based on the area of CB30, with bottom depth at 4303.88m. It aims at exploring buried hill reservoir. Different drilling fluid system were selected in relation to different formation characteristic. In the upper formation, polymer drilling fluid is used, and the lower, no-solids polyalcohol drilling fluid is used. Lost circulation was encountered when reached at 4220.44m, losing rate is 48m³/h. Subsequence, no-plugging and protecting reservation drilling technology was applied to achieve the fourth drilling. Results reveal polymer drilling fluid has strong shale inhibition, cutting transportation, hole stabilization abilities. No-solids polyalcohol drilling fluid has better high-temperature resistance and lu-bricity, sloughing prevention and formation damage prevention.