Some polymer materials with negative Poisson’s ratio had been developed

but negative Poisson’s ratio effect was seldom observed for rockmass. Based on the first underconstruction undersea tunnel as engineering background

a brief introduction of indoor test on the rockmass specimens was given; and negative Poisson’s ratio effect was observed during test. A new microcosmic deformation mechanism—lattic-rotation model which could interpret the observed test phenomenon was proposed; and a credible data proceeding method to obtain the actual value of Poisson’s ratio was proposed too.