Preflight ground flat-field calibration is significant to the development phase of space astronomical telescopes. The uniformity of the flat-field illumination reference source seriously decreases with the increasing aperture and the telescope's field of view, directly affecting the final calibration accuracy. To overcome this problem, a flat-field calibration method that can complete calibration without a traditional flat-field illumination reference source is proposed on the basis of the spatial time-sharing calibration principle. First, the characteristics of the flat field in the spatial domain taken by the space astronomical telescope are analyzed, and the flat field is divided into large-scale flat (L-flat) and pixel-to-pixel flat (P-flat). They are then obtained via different calibration experiments and finally combined with the data fusion process. L-flat is obtained through star field observations and the corresponding L-flat extraction algorithm, which can obtain the best estimation of L-flat based on numerous photometry samples, thereby effectively improving calibration accuracy. The simulation model of flat-field calibration used for accuracy analysis is established. In particular, the error sources or experimental parameters that affect the accuracy of L-flat calibration are discussed in detail. Results of the accuracy analysis show that the combined uncertainty of the proposed calibration method can reach 0.78%. Meanwhile, experiments on an optic system with a Φ142mm aperture are performed to verify the calibration method. Results demonstrate that the RMS values of the residual map are 0.720%, 0.565%, and 0.558% at the large-, middle-, and small-scale, respectively. The combined calibration uncertainty is 0.88%, which is generally consistent with the results of the accuracy analysis.