Linear equation between peak stress and Young's modulus with confining pressure is obtained from triaxial compression for limestone under different confining pressures. The relationships between strain energy and strain under three confining pressures are fitted by using stress-strain curve

which can obtain the energy value of limestone in anyone axial strain. Several energy changing equations of several stages in the compression process are analyzed

so as to explain the more several rock failures under higher confining pressure in consideration of the rock sample doing work to hydraulic oil. Based on crack extension degree affects the value of the Young's modulus

the concept of crack development coefficient F is proposed; and the empirical formula to strike F using ? under the three confining pressures is given; and the real rock mechanical parameters of various stages during the limestone compression are got; the plastic platform appearing near the peak intensity under high confining pressure with changing tendency of F is explained.