The mechanical features based on unloading mechanical parameters are not accordant with the mechanical characteristics of rock under unloading condition; in order to construct strength criterion which is widely used and accordant with characters of unloading rock

the work and results are carried out as fellows. Firstly

different initial damage levels between unloading experiment of rocks and underground engineering under excavation unloading conditions are discassed. Secondly

the internal relations between elastic strain energy

the second invariant of stress deviator

the second invariant of stress tensor and Poisson’s ratio are explored. Thirdly

the internal relations between elastic strain energy and strength criterion are also explored. Lastly

based on elastic strain energy

the mechanical features of rocks under unloading condition are studied by using loading mechanical parameters. The experiment results show that existed strength criterion is possible to violate the first thermodynamic law or the law of conservation of energy. It is suitable to increase or decrease for the limited bearing loads of rock under the range of unloading condition based on elastic strain energy

and the mechanism of above-mentioned result is analyzed.