Laser additive repairing technique was performed to repair the damaged aeroengine turbine blades made of K465 nickel-based superalloy. The cracks characteristics and cracking mechanism of the laser additive repaired K465 superalloy were investigated. The cracks were eliminated successfully by taking some effective measures. The results showed that the cracks originated from the heat affected zone of the laser additive repaired specimens, and extended to the repaired zone along the grain boundaries. The continuous liquid film was derived from the liquation of larger γ′ particles at the grain boundaries and γ-γ′ eutectics appeared during grain boundary liquation. Stress field simulation results obtained using the Ansys software demonstrated the existence of large thermal stresses at the zone between the substrate and the molten pool. Through preheating the substrates synchronously and adopting the optimized processing parameters, the laser additive repaired specimens with the structure of single track and multi layers without any cracks were obtained.