In December 2023, the project “Construction of Regional Deep-Argo Observation Network” sponsored by Laoshan Laboratory deployed a domestically-produced HM4000 profiling float with the maximum profiling depth of 4 000 m (the World Meteorological Organization number is 2902895) in the Philippine Sea, which was equipped with an RBRargo³ deep 6k Temperature-Conductivity-Depth (CTD) sensor produced by RBR, Canada. It was found that the salinity observation data reported by the float exhibited a systematic deviation compared to the shipboard CTD and climatological salinity. In order to correct the salinity data of the float, the conductivity slope of the RBR CTD was calculated by using bottle salinity measured by the Autosal 8400B salinometer and salinity measurements from the shipboard CTD cast. Salinity profiles of the float were then calibrated, and the calibrated salinity was found to be basically consistent with the nearby float and the climatological data. With the implementation of the “Construction of Regional Deep-Argo Observation Network” project, an increasing number of domestically-produced deep Argo floats will be deployed. Compared to the Core Argo floats that measure temperature and salinity profiles in the upper 2 000 m of the ocean, Deep-Argo requires higher accuracy to resolve smaller variations in deep waters. Currently, technical problems are still found in deep CTD sensors, and improper handling and operation during storage, transportation, and usage of some floats and sensors are inevitable, resulting in large errors in the observations, especially the salinity data. Therefore, this study proposes a method of calibrating Deep-Argo floats’ observation data using in-situ shipboard CTD cast, which can provide essential technical support for quality control of the Deep-Argo floats.