High precision and stability are demanded for mainboard as a core construction of high resolution space camera, thus, traditional measurement methods could not match the application requirements. According to the structure and specification of the mainboard used in a large aperture camera and the capability of measurement equipment, horizontal placement was adopted for flatness test of mainboard datum plane using gantry coordinate measuring machine (CMM). The concept of quasi-kinematic mount was introduced, the mainboard was supported by 3 inverted Bipods to avoid clamping and assembly deformation, a specialized measurement structure was designed and manufactured as well. The position and magnitude of unloading force were confirmed by numerical simulation based on ABAQUS and Matlab with an index of datum plane's flatness error (PV). The results show that using designed measurement structure datum plane's flatness error is less than 2 μm. This method promotes the capability of CMM for measurement of large precise objects and has widely application prospect.