The invention relates to an outside-sealed displacement adjusting mechanism for a parallel-connection spray pipe, and relates to a combined engine tail spray pipe. The outside-sealed displacement adjusting mechanism for the parallel-connection spray pipe is provided with an actuator cylinder, a connecting rod, a rocker arm, a fixed hinge, a long sliding block, a long sliding block cam shaft, a spray pipe side plate, a spray pipe intermediate diaphragm, a spray pipe upper wall surface, a splitter plate and a spray pipe lower wall surface. The actuator cylinder can move left and right; the rocker arm rotates around the fixed hinge; a through hole is formed in the lower end of the rocker arm; the long sliding block cam shaft can move in the hole; a side plate slot is formed in a spray pipe side wall; a through hole is formed in the center of the slide plate slot; the long slide block can slide in the side plate slot; the length of the long sliding block is larger than the length of the through hole of the side plate slot, so that during a sliding process of the long sliding block, the sliding block can always cover the through hole of the side plate slot, and a sealing effect is achieved; the long sliding block cam shaft is arranged at positions, on two sides of the side plate, on the long sliding block; the long sliding block cam shaft on the outer side of the side plate is connected with the rocker arm through the through hole; and the long sliding block cam shaft on the inner side of the side plate is connected with the splitter plate through a splitter plate sliding chute.An air flow state of a spray pipe outlet can be improved, and better thrust performance is obtained.