The invention discloses a method for improving device installation precision of a spacer to be brushed. The method specifically comprises the following steps: fixing a welding spacer and a regulatingspacer on a substrate faceplate of a device with the spacer to be brushed; regulating the device so that a bearing thereof bears the pressure and an end flange satisfies the design requirements; fixing a decreased value measuring fixture on a to-be-measured position of the substrate faceplate of the device; measuring a first height value; installing a foundation bolt; measuring a second height value; unscrewing the foundation bolt, and regulating and knocking the regulating spacer according to the first height value and the second height value; and screwing the foundation bolt. The decreased value measuring fixture is simple in structure, convenient to use, and suitable for regulating the installation precision of all devices with the spacer to be brushed, and is capable of accurately measuring the decreased value of the device substrate after the foundation bolt is installed, wherein the measurement precision is up to 0.01 mm of the measurement level. The regulating method is capableof guaranteeing that the substrate precision is not affected by the installation of the foundation bolt.