In this work, we use finite element simulations to investigate the influence of cooling water temperature on the axial growth rate of the large single crystal diamond with the convex shape catalyst under high pressure and high temperature (HPHT) conditions. The simulation results show that adjusting the axial cooling water temperature can change the axial temperature gradient and the convection velocity in the catalyst, which lead to the different axial growth rate of the diamond crystal. The different axial growth rates lead to the change of crystal morphology. Simulation results are confirmed by our synthesis experiments. Morphology and structural properties of the synthesized diamonds are characterized by optical microscope and Raman spectrum. This result not only provides a theoretical explanation for the regulation mechanism of cooling water on the axial growth rate of diamond, more importantly, adjusting the axial cooling water can further improve the morphology of the synthetic diamond, which is of great guiding significance in the commercial production of diamond.