Abstract: The marine gas reservoirs in Leikoupo Formation of the Western Sichuan Gas Field have extreme burial depths and complex geological conditions, and the four-stage casing program that was previously used cannot meet the requirements of a faster drilling pace and cost-effective development. By analyzing the engineering geological characteristics and on the basis of ensuring the specific layer drilling in the Leikoupo Formation, the positions of mandatory sealing points were optimized according to the results of formation three-pressure profile and borehole stability studies. Based on that, an ultra-deep highly deviated well three-stage casing program was designed. Combined with the lithological characteristics of this formation, we optimized highly deviated borehole trajectory and directional bits and support tools so that that would work well in high-abrasive strata. Further, we developed both an anti-collapsing drilling fluid with compound salt strong inhibition polysulfide and a high acid solution polysulfide drilling fluid with strong plugging performance. The three-stage casing program for ultra-deep highly deviated well and the associated drilling technologies was tested in Well PZ4-2D, which effectively solved the problem of borehole instability in long open-hole complex formation. The total depth reached 6 573.77 m, in a drilling period of 199.3 days, with an average ROP is 3.53 m/h. The drilling speed increased more than 40%. The results of field test showed that the design of three-stage casing program was scientific and reasonable, and the associated drilling technologies were effective in increasing the drilling rate significantly, which provided technical support for the subsequent drilling of highly deviated wells in the marine gas reservoirs of the Western Sichuan Gas Field.