In order to achieve high efficiency and high quality plasma cladding of directionally solidified DZ125 alloy,under the condition of water cooling,Eu Tro Loy16006 Co-based alloy powder cladding layer is prepared on the surface of DZ125 alloy by plasma arc spray welding to study the effect of different cooling conditions on the microstructure growth and hardness of cladding layer. The results show that the fine columnar crystal microstructure forms at the bonding interface of the base material,and the microstructure changes from columnar crystal to equiaxed crystal when the cladding layer thickness increases.The growth height of columnar crystal under water cooling condition is higher than that under air cooling condition. The hardness of the columnar crystal is similar to that of the base material,and the hardness of the equiaxed crystal is lower than that of the columnar crystal. The hardness of the equiaxed crystal close to the surface of the cladding layer is the minimum. The cladding layer with the same hardness as the base material obtained under the water cooling condition is wider than that under the air cooling condition.