In order to realize high-precision, low-cost measurement of optical refractive-index homogeneity of large-size optical material, an interference absolute testing method based on subaperture stitching technique is proposed and a measurement system comprised of a Zygo interferometer, a sophisticated five-dimensional adjustment air-floatation platform, subaperture stitching software and a computer is developed. The optical component under test is placed on the five-dimensional adjustment platform and all subaperture areas of the component can be measured precisely by moving the platform. Then optical-homogeneity distribution of the whole optical component is calculated out automatically by subaperture stitching software. In the experiment, a 300 mm quartz optical component is measured in stitching mode of eight subapertures by using a 180 mm interferometer. The stitching result is compared with the direct measurement result obtained by using full-aperture interferometer. The relative error of wavefront peak-to-valley value is 0.21%, the relative error of optical homogeneity value is 0.23%. This precision can be compared with the precision of directly measured result. The practicability of subaperture stitching technology of plane-type wavefront measured by using absolute test method is realized. The whole system integrates optics, mechanics, electrics and computer together and can be operated conveniently with high accuracy.