The spatial resolution and modulation transfer function (MTF) are important parameters for image-quality evaluation of high-spatial-resolution satellite optical sensors. They objectively reflect the imaging quality of a remote-sensing imaging system. We present a method using a radial target for spatial-resolution measurement that employs a large-area target with uniform reflectivity. This method depends on the relationship between the target modulation degree, object modulation degree, and image modulation degree in the pupil of the sensor. It can accurately determine the on-orbit MTF value of a spaceborne remote-sensing imaging system without atmospheric effects as well as the MTF value of the atmosphere. Experimental results show that the proposed radial-target method can simultaneously determine the spatial resolution and the on-orbit MTF value of the spaceborne remote-sensing imaging system. The real-time MTF value of the atmosphere is 0.7519. The difference between the on-orbit MTF value measured by our radial-target method and that measured by the knife-edge method is less than 5%. The proposed method can provide on-orbit image-quality evaluation of high-spatial-resolution satellite optical sensors.