Laser rapid forming (LRF) is a new manufacturing technology, which has been developed on the basis of multi-layer laser cladding. In the LRF process, the microstructure has important effects on the mechanical properties of the partsbut the control of microstructure is a problem. In this study, the influences of crystallography orientations of substrate and profile of solid/liquid interface on microstructure were discussed. Further, with the combining of the columnar to equiaxed transition (CET) model during alloy solidification, the growth law of microstructure of Rene88DT alloy was established. It has been found that the temperature gradient was lowest and the solidification velocity was greatest at the solid/liquid interface of the tail of molten pool, and hence the CET occurs easily at this position. The temperature measurement system of molten pool was developed by using a two-color infrared thermometer in this study. With the measurement of temperature gradient of the tail of molten pool by using a two-color infrared thermometer, the process parameters of laser multi-layer cladding were optimized. Finally, directional solidification even single crystal was achieved in laser multi-layer cladding.