Ti6Al4V alloy thin wall is prepared by using laser melting deposition, and the influences of process parameters on the size stability of molten pool and microstructure growth characteristics of deposited layers are studied. The temperature field of molten pool and cooling rate of melt under different process conditions are calculated based on finite element simulation method. The results show that the molten pool size varies significantly with the change of heat dissipation condition of substrate in the process of melting deposition of the first few layers. The heat input of each layer is optimized, which can realize the accurate control of stability of molten pool size. Compared with laser power, the influence of scanning speed on directional growth characteristic and grain size of deposited layer is more remarkable. With the change of scanning speed, the microstructure of deposited layer can easily transform from columnar crystal to equiaxial crystal. Under these experimental conditions, the directional growth columnar crystal can not be formed unless the cooling rate of molten pool is higher than 135.3 ℃/s.