In engineering practice, the prevention of early-age cracking in massive concrete structures is of great importance to their serviceability during the whole life cycle. From the scientific viewpoint, this engineering concern requires the control of the early-age performance of concrete structures. Following earlier research projects against the background of the Hong Kong-Zhuhai-Macao Link, the focus of this work is to obtain insights into the evolution of the early-age behaviour of precast concrete in an immersed tunnel. To this end, a full-scale test was performed, from which the behaviour of early-age concrete could be observed directly. After validating the constitutive model developed with the test results, the early-age performance during the entire fabrication process of the precast concrete immersed tunnel is evaluated numerically. It was also found that stress relaxation plays a major role in stress development in the immersed tunnel, although the thermal strain is the main source of early-age stresses. This in-depth investigation resulted in a comprehensive understanding of the early-age behaviour of an actual precast concrete immersed tunnel. What is more important, the early-age performance of concrete structures can be accurately evaluated and further adjusted or controlled with the help of the validated numerical modelling, which is no doubt beneficial for the control of early-age cracking in massive concrete structures in engineering.