This paper systematically analysis the stress-strain behaviour of mud stone under different confining pressure. The relationship between the mechanical behaviour of mud stone and confining pressure is established, and the curves of stress-strain relationship of mud stone are fitted. The results indicate that the failure pressure and the elastic modulus increase with the increasing of confining pressure conforming to the linear function. The stress- strain of mud stone is divided into four different stages which are fracture closing, elastic strain, fracture expanding and plastic strain stage. The stress-strain behaviour in different stage has different function law, the stage of fracture closing can be fitted by polynomial function, the stage of fracture expanding can be fitted by hyperbola function, the fitting result is ideal.