Abstract: To meet the challenge of the Chinese Continental Scientific Drilling Project (CCSD),it is necessary to develop ultra-high temperature drilling fluid.This paper chemically created the best thickening agent which is VIS-H by comparing the principle of several agents,then determination of proper bentonite dosage;selected the film forming agent LockSeal,filtrate loss controller SHR,then further screening the other agents and built the ultra-high temperature water-based drilling fluid system.Evaluated comprehensive performance of this system in laboratory,the results shown that this system have good temperature-resistance at 240℃;it may give us some guidance for future ultra-deep wells scientific drilling.