An automatic opening/closing mechanism is disclosed. It includes a coil spring, a pushing member, a driven member and a contact board which are disposed in a cylinder under a decorative article. A shaft is disposed at a center of the coil spring. One end of the shaft is connected with one end face of the pushing member. The pushing member has a disc-like shape. The other end face of the pushing member is disposed with several slope teeth which are slidably engaged with several slope recesses of one end of the driven member. The other end of the driven member abuts against the contact board. Two electric contacts are disposed beside the contact board. The power of coil spring is gradually released to rotate the pushing member so as to upward push the driven member for lifting the contact board to contact with the electric contacts and close the circuit. At this time, a bulb or a music box is turned on to emit light or music. After the power of the coil spring is exhausted, the driven member is no more pushed by the pushing member and is engaged therewith to automatically shut off the electric power.