The utility model discloses a locking structure for prolonging a support arm of a mobile phone support, which comprises a support cover, a left clamping arm, a right clamping arm, a support arm, a vertical arm, a stretching piece, a support arm, a gear, a rebound spring, a clamping connector and a bottom shell, and is characterized in that the stretching piece is arranged on the support arm and connected with the support arm, and a hook is arranged below the support arm; the clamping connector is arranged and located below the hook of the supporting arm, when a mobile phone is clamped, the hook of the supporting arm is clamped into the clamping connector to be locked, the stability of the whole support is improved, and especially when a large-size mobile phone is clamped, the supporting arm drives the stretching piece to move downwards, the length of the whole supporting arm is increased, the clamping stability of the mobile phone is guaranteed, and the support is suitable for mobile phones of any type. The two-way transmission gear controls the left clamping arm and the right clamping arm in a linkage mode, and the clamping performance of the clamping arms is more stable.