Reef-building corals play key roles in maintaining marine biodiversity and forming coral reefs. Understanding the early growth of corals is crucial at present for laboratory experiments regarding coral ecological recovery. Here, the authors designed an auto-calibration balance system to measure the 24-week early growth of eight Acropora formosa branches, which are suspended in seawater. The auto-calibration process is carried out before each measurement by loading a standard weight for setting the balance to zero, which solves the zero shift problem of ordinary balance due to continuous work in-situ. Through linear regression analysis, the authors' measurement results reveal that the newly segmented coral branch shows an increasing growth rate in early growth. Within a certain range, an A. formosa branch with larger initial weight, comes with greater growth rate increasing, while the branches with an initial weight between 3.5 g and 4 g show the best growth potential during the whole measurement. Meanwhile, these data also reveal that the balance has high stability and that the system can be used to monitor the long-term growth of reef-building corals in seawater for laboratory research, providing strong support for coral ex situ conservation research, and is a strong impetus for global ecological restoration of reef-building corals.