Different As-soak time is applied during InSb-like interfaces growth of InAs/AlSb superlattices on GaAs(100) substrates. The interface roughness is studied by grazing incidence X-ray reflectivity. The reflectivity curves are simulated by standard software and the rms roughness of the interfaces is obtained. It was hown that the sample with As-soak time of 20 seconds has the most smooth interfaces. By analyzing the microscope images of the samples, we suggest that In-rich interfaces will be formed with too short As-soak time and AlAs-like interfaces are obtained with too long As-soak time, and in hoth cases the interface will be cearsened. Grazing incidence X-ray reflectivity is also recommended as a powerful tool for assessing the structure of superlattices.