The dependence of temperature and velocity distributions on rotation speed during KDP crystal growth process were numerical simulated via finite volume method and the Fluent software. It revealed that at the rotation speed of 9 r/min, the temperature value nearby the crystal was relatively higher, and the temperature distribution was stable, which resulted in single capping and slow capping speed. When rotation speeds reached 55 and 77 r/min, the temperature value nearby the crystal was comparatively low, which increased the supersaturation and accelerated the capping speed. The rotation speed between 9 and 50 r/min could bring temperature turbulence, and consequently led more capping. The capping speed was relatively moderate. The simulation result proved the rotation speed of 77 r/min was the optimum speed.