The invention discloses a secondary locking mechanism of an underwater horizontal connector installing tool. The secondary locking mechanism comprises a gear rack, a gear, a gear fixing shaft and a locking head, wherein the gear is fixed on the gear fixing shaft; when the gear rack is meshed with the gear, the gear fixing shaft rotates under the action of the gear; a compression spring is arranged inside a cavity of the gear fixing shaft; the locking head is arranged at one end part of the gear fixing shaft, and extends out of the gear fixing shaft; and the locking head can move along the axial direction of the gear fixing shaft under the action of elasticity of the compression spring. Without assistance of an ROV (remotely-operated vehicle), automatic locking is achieved in the installation process; and the locking head adopts a special columnar structure, so that the firmness after locking is ensured and the reliability of a whole connector is improved.