The geometrical model is used to investigae the orientation of dendrite growth in a laser weld single crystal and the variation of the distribution of orientation zones when angle ξ between the laser scanning direction and the 100 orientation in (001) surface changes. With the increase of ξ, the 0 zone increases whereas the 010 zone shrinks. Meanwhile, the growth velocities of dendrite are different in different orientation zones inside the weld pool. The undercooling region is then constructed on the basis of the calculation results. It is found that the undercoolings are larger at the boundaries than in the grains for all orientation zones, which makes the boundaries vulnerable to the CET transition and production of the new grains. Moreover, with the increase of ξ, the abilities to form the new grains are different on the two sides of the weld pool. For comparison, the DD3 single crystals are laser welded and the experimental results are in good agree meent with theoretical ones.