The mechanical and acoustic characteristics of yellow sandstone under small-size effect were studied. The rock strength，deformation，elastic constants(elastic modulus，Poisson's ratio)，failure morphology and energy conversion law under small-size effect were discussed. The relationships between the acoustic emission energy，the count，the peak frequency and the size were analyzed. As the size increases，the strength decreases linearly，and the radial strain extends from the oblique to the horizontal direction and exhibits a tendency to decrease first and then to stabilize，and the axial strain gradually reduces. Body strain increases first，then stabilizing and then decreasing. The rupture range decreases gradually. The shear angle decreases first and then increases. The elastic modulus and Poisson's ratio show an increasing trend. The energy after the peak shows a release law of accelerating first，then stabilizing and then decreasing. With the decrease of size，the dissipated energy and the total energy accumulated before the peak show an increasing trend，the dissipative energy acceleration phase and the deceleration phase are gradually decreasing，and the stabilization phase is gradually extended. There is a tendency of “closed-cup” between energy，elastic energy，release energy and surplus energy all show a decreasing trend，acoustic emission energy and time show a“double peak”image. The emission count and energy show“horn” expansion，while high energy，high meter，high peak(three high) can predict the impending destruction of the rock；as the size decreases，the total energy of the acoustic emission appears after the peak interval is counted. The total energy of the acoustic emission is proportional to the size. The characteristics of acoustic emission peak frequency is obvious partitioning.