To perfect the axiom system of classical mechanics, a new fundamental principle,named Zero-principle, is established on the basis of three foundation-stones: 1) the concealing-displaying model, 2) the mode-assumption of displaying force,3）the μ-proof. The conclusion of the principle is summarized into three equivalent ‘zero-force systems’. The new principle makes a series of important contributions: 1）Raise Newton’s 2nd law into a theoretical principle, so that it is no longer fully experiment-based. 2) Make D’Alembert’s principle become a principle independent of Newton’s 2nd law.3）Establish a reformed principle,named principle of minimum freedom, including Gauss’ priniciple of least constraint as a particular case. As a consequence,the so-called Z-paradox in Gauss’ principle is solved. 4）Provide a brief natural proof of new Hamilton’s principle of ‘least action’. 5) After generalizing,, the Zero- principle is applied to the impulsive condition.