From the proposal of EPR Paradox to the experimental testing of Bell's Inequality, Hidden Variable Theory has realized the transition from metaphysical dispute to testable scientific theory. The evolution of the Theory shows that the "testability" of hypothesis is not invariable, but keeps changing with historical conditions, which means that it is problematic to consider "testability" as a purely analytical concept as it is a category that is both analytical and synthetic.In order to better interpret the real science history, it is necessary to get rid of the traditional analytical understanding and to re-understand "testability" from a historical perspective.