Antarctic coastal polynyas provide extremely dense water to the global abyss, but the dense water formation process has been poorly monitored in winter. This study developed a new tethered profiler to realize time-series observations of the water column in the Cape Darnley Polynya, East Antarctica. The system successfully obtained temperature and salinity profiles at 10-day intervals from March to November 2017. From March to April, significant cooling and vertical mixing started, while stratification collapsed. Salinity increased rapidly from April to late-May and then gradually increased until October. Salinity development was largely consistent with the cumulative salt increase due to sea-ice production at the initial stage, but not at latter stages, indicating the influence of cross-shelf exchange. These results highlighted the potential of the measurement platform to fill the remaining gap in the global ocean monitoring network.