The invention relates to a unit brake cylinder parking brake mechanism and belongs to the technical field of rail transit vehicles. The unit brake cylinder parking brake mechanism comprises a cylinder body with an extension section, an axially movable piston tube is supported in the cylinder body and is filled with an extension control shaft assembly forming axial constraint with the piston tube, a thrust support forming axial constraint with the piston tube sleeves the extension section of the cylinder body, a shifting fork with a power arm is arranged on one side of the cylinder body in a hinge support manner, and two sides of a fork entry end, far away from the power arm, contact with bosses on two sides of the thrust support respectively. When the shifting fork is at a release position, the distance from the hinge support center to the contact distance between a fork entry and each boss is longer than that from the hinge support center to the contact distance between the fork entry and each boss when the shifting fork is at the brake position. The unit brake cylinder parking brake mechanism has the advantages of compact structure, high suitability and more reliable brake performance.