Abstract: This paper investigates laser cladding the sealing surface of high parameter petrochemical valves with 5kW transverse flow CW CO2 laser.The base metal of valves is various,and the cladding powder is CoCrWB or NiCrFeBSi powder.A 2mm～3.5mm thick layer of laser cladding with smooth surface and non flaw can be made,based on solution of crack problem of cladding layer by a single scanning and thick cladding.Various elements and their interaction which influence the formation of crack in laser cladding layer,especially of thick layer,and control of crack are discussed and analyzed according to the experiments.The thought of building a model of stress judgement to judge the formation of crack in cladding layer is put forward.