For the pier segment of the segmental precast bridge, the diaphragm is often casted in place to reduce the lifting weight. Considering the volume of the diaphragm, it belongs to massive concrete,in which concrete hydration heat may cause excessive tensile stress in some parts. Without right control measures often lead to cracking of the precast segments during the construction phase. Based on three-dimensional transient temperature field theory, numerical simulation with ANSYS was performed, to investigate the distribution law of hydration heat temperature and stress. The analysis shows that, the tensile stress at the outside of the webs is sufficient to lead to concrete cracking, longitudinal micro-cracks along the webs is also observed during the hydration heat test. With the distribution of temperature and stress obtained from numerical simulation, a series of control measures were proposed and adopted to the Fourth Nanjing Yangtze River Bridge approach project, which were proved to be feasible and effective.