Selecting proper imaging intervals where the ship rotation vector is constant is a very simple method to obtain high-quality ship inverse synthetic aperture radar (ISAR) images. However, existing imaging interval selection methods are usually based on traversal or iteration, which means that they are time-consuming and complex. Thus, to realize imaging interval selection simply and effectively, a modified imaging interval selection method based on joint time-frequency analysis (JTFA) is proposed in paper. Compared with the traditional range-Doppler algorithm (RDA), the proposed method just has one added step of JTFA. Real data processing results validate the effectiveness and superiority of the proposed method.