In order to evaluate the modulation transfer function (MTF) of different subpixel imaging modes of spaceborne linear charge-coupled device (CCD), a quantitative assessment method based on MTF which can better characterize image quality is proposed. The MTF of subpixel imaging modes of linear CCD is derived. Compared with conventional imaging mode by a single linear CCD, analysis shows that, theoretical limiting resolution values of 1/2, 1/3, 1/4 CCD-pixels staggered subpixel imaging modes by simultaneously using multiple uniform linear CCDs are improved by 86.2%, 88.78%, 89.54%, respectively, MTF values at normalized spatial frequency of 0.5 are increased by 0.1679 (41.43%), 0.2026 (49.99%), 0.2151 (53.07%) respectively, and modulation transfer function areas (MTFAs) in the normalized spatial frequency range of (0, 0.5) are enhanced by 10.09%, 12.08%, 12.77%, respectively. Simulation experiment on Matlab platform is performed. Gray mean gradient (GMG) and energy of Laplacian (EOL) are utilized to evaluate the image quality performance of three subpixel imaging modes objectively and the results demonstrate the validity of this method. The proposed method has some reference value to the design of linear CCD subpixel imaging system in remote sensing.