Provided is a miter gate back drag rod prestress adjusting method and device. The device comprises a jacking cross beam, wherein a screw at the end of a back drag rod is sleeved with the jacking cross beam, a tool nut is arranged on the screw, a back drag rod nut is arranged at the screw section between the jacking cross beam and a back drag rod support, a stress application device is further arranged between the jacking cross beam and the back drag rod support and is composed of two jacks, and the jacks are respectively connected with a hydraulic pump source. The adjusting method comprises the steps of adding the stress application device, measuring pull force and stress of the back drag rod, adjusting prestress and the like. By means of the miter gate back drag rod prestress adjusting method and device, the defect that the back drag rod is completely loosened to determine a zero value, then the prestress of the back drag rod is adjusted to be a required value and accordingly a construction period is long when miter gate back drag rod prestress adjustment is performed in the prior art is overcome, the prestress of the back drag rod is not required to be loosened to be the zero value during measurement, adjustment is directly performed based on original prestress, and the construction period is shortened and operation is simplified while the adjusting effect is ensured.