The invention provides a predetermined water content determining method for a mixture compaction test. The predetermined water content determining method comprises the steps that mineral powder and screened aggregate with screen hole diameter of 0.6 mm are mixed, and the upper limit and the lower limit of the predetermined water content are calculated according to the following: the upper limit of the predetermined water content is (shown in the description), and the lower limit of the predetermined water content is omega down = Nc * omega c + (Nf + C) omega * omega fcr. The predetermined water content determining method for the mixture compaction test can be used for determining the predetermined water content of the compaction test of the mixture prepared by blending multiple materials, and the upper limit and the lower limit of the predetermined water content of a cement stabilized macadam compaction test are determined, so that the compaction test can be performed in the more accurate water content range of the mixture prepared by blending multiple materials, the working amount of the compaction test can be saved, and more accurate compaction results can be obtained.