The physical mechanism of crack formation in the process of laser cladding is analysed by theoretical derivation and the influence of laser spot diameter on crack defect is researched. The powder of Ni60 is cladded on the surface of base material (40Cr) by experiment and the characteristic of crack is analysed by observing the distributions of crack with optical microscope. The results show that the crack ratio of laser cladding layer is increasing gradually with the increase of spot diameter, in addition, the depth and width of crack are increasing obviously.