The invention provides a gap assembly method based on instrument compartment inertial measurement unit platform mounting. The gap assembly method comprises the following steps that (a) a K-shaped beamis placed on a middle frame, and the middle frame is placed on a riveting assembly fixture platform; (b) the inertial measurement unit platform is placed on the middle frame and the K-shaped beam, and the inertial measurement unit platform and the locating fixture are fastened; (c) a gap A between the inertial measurement unit platform and the middle frame and a gap A between the inertial measurement unit platform and the K-shaped beam are measured; (d) gaskets are added between the inertial measurement unit platform and the middle frame and between the inertial measurement unit platform andthe K-shaped beam; (e) the inertial measurement unit platform is in threaded connection with the middle frame and the K-shaped beam; (f) a bolt between the inertial measurement unit platform and the locating fixture is loosened, and a gap B at the threaded connection part of the bolt between the inertial measurement unit platform and the locating fixture is measured; and (g) gasket adding adjustment is carried out at the parts where the bolts are connected between the inertial measurement unit platform and the middle frame and between the inertial measurement unit platform and the K-shaped beam for the second time. The one-time assembly success rate is increased, the assembly efficiency of the inertial measurement unit platform is improved, the assembly quality of the inertial measurementunit platform is ensured, and the inertial measurement unit platform meets the design and operational requirements after being assembled.