An underwater weapon will undergo stronger impact when it is in the incipient transient process of the entry-water, and this will cause serious harm to its structures and the carried instruments. It is very important to increase the reliability of the underwater weapon through researching the water entry impact of underwater weapon. In this paper, the numerical simulation of the vertical water-entry with zero attack angle at low speed for a missile is carried out based on the commercial software Fluent. The numerical simulation method of the water-entry at low speed for a column missile model is established using the technologies of dynamic mesh and UDF, and is validated by the experimental picture. The result shows that the numerical simulation accords with experimental results preferably at low speed for a missile with laminar flow model, however, the numerical simulation of the turbulence model shows an obvious error. So, the suitable method for low speed water-entry is not the turbulence model but the laminar flow model.