Water has softening, dissolving and water wedging effects on rocks. To study the energy mechanism of rock with different moisture contents, the conventional triaxial compression tests on sandstone with five moisture contents are conducted using MTS815 rock mechanics test system. The results show that, with the increase of moisture contents, the increase rate of total energy absorbed by rock and its amount decrease. The increase rate of elastic energy decreases with the increase of moisture content, but their release rates differ slightly. The energy storage limits decrease with the increase of moisture content. The dissipated energy causing rock failure decreases as the moisture content increases, but the increase rates of ante- and post-peak dissipated energies are roughly identical. The proportion of dissipated energy to total energy can reflect the status of internal damage of rock, the variation of the proportion with time can be divided into four stages, i.e., gradual increase, decline, stable increase again, sharp growth. With the increase of moisture content, the concentration degree and intensity of acoustic emission energy gradually decrease. The total acoustic emission energy decreases as the moisture content increases. It is shown that the energy storage capacity and strain energy release ability of rock reduce with the increase of moisture content, meanwhile its plasticity is enhanced and brittleness is weakened.