A fixture adapted to be mounted on a stand for supporting a guitar includes a seat having two compartments, two moving blocks respectively and pivotally received in the two compartments and a resilient member received in the seat to provide recovery force to the two moving blocks after the two moving blocks are pivoted, a cap having two arcuate slots to respectively communicate with the two compartments and two arms to securely connect to a corresponding one of the two moving blocks such that when the arms are moved closer to each other to securely clamp the guitar due to limitation of the two arcuate slots and a weight of the guitar, the two moving blocks are pivoted accordingly and the recovery force from the resilient member is able to push the two moving blocks as well as the two arms back to their original positions.