The last two decades have witnessed a dramatic development of the satellite interferometric synthetic aperture radar (InSAR) technology in both the theoretical methods and operational platforms. The propagation errors of SAR signal in the ionosphere is one of the problematic issues for the low-frequency SAR systems, such as L-band and P-band, and can also seriously degrade the accuracy of InSAR. Recently, there has been renewed interest in the range split-spectrum method for the ionopheric correction after some critical issues are resolved. In this study, we used the ultra-fine observation mode (80 MHz) of the Advanced Land Observation Satellite-2 (ALOS-2) phased array-type L-band synthetic aperture radar-2 (PALSAR-2) data over the 2016 Kumamoto earthquake to evaluate the performance of an asymmetric split-spectrum method for the ionospheric correction. The presented works demonstrate the effectiveness of two types of the asymmetric split-spectrum method for mitigating the ionospheric artifact in InSAR, as compared with the conventional split-spectrum method. Additionally, this study provides practical insight into the ionospheric correction strategies of the scheduled NASA-ISRO synthetic aperture radar (NISAR) mission.