Because the current methods for determining specific yield of undisturbed soil are not precise, aimed at designing a new determinator and proposing a new test idea and its calculation method, the research was conducted. First, the structure of a new determinator and its operational process for specific yield were proposed. Second, based on the analysis of infiltration and water release process of soil and the definition of specific yield, the formula presented by physical property parameters of medium and penetration liquid was obtained for computation of specific yield. The analysis results and examples indicated that the determination process of the determinator designed is simple. The seepage process was approximately the same as the practical flow pattern, and the formula has high accuracy.