In the process of growing 300 mm diameter silicon single crystal by Czochralski method, uniform diameter is the key to obtain high quality silicon single crystal. It is found in production practice that too high pulling speed causes the phenomenon of twisting crystal at uniform diameter growth stage, which leads to the crystal line fracture and then crystalloblast, and it is unfavorable to uniform diameter growth. In this paper, the origin of the twisting crystal in the growth of 300 mm silicon single crystal was analyzed by the combination of numerical simulation and theory analysis. For varying pulling speed, the correlation between crystal diameter and melt temperature distribution was established, the affecting factors on twisting crystal were obtained. The results show that with the increase of pulling speed, the supercooling zone is generated on the free surface of the melt, and continue to expand. The generation of supercooling zone is the main reason leading to twisting crystal. A method for identifying the maximum stable pulling speed is presented based on the numerical simulation of the finite element thermal field, and the method of varying the rotation speed of the crystal is proposed to improve the free surface temperature distribution of the melt, so as to avoid the crystal twisting. The results provide a guidance to the thermal field design of large diameter silicon monocrystalline growth.