The concept of relative dilution ratio of the substrate and its calculation formula are suggested in studying multi-pass laser cladding dilution ratio. And the heat effects of the first pass on the second pass and relative dilution ratio of the substrate are discussed in the respective of the cooling time, overlapping center offset and overlapping ratio. For the GH4033 substrate, it takes 360 s to decrease the heat effect of the first pass on the second pass markedly and to make relative dilution ratio of the substrate near to zero. The larger the overlapping center offset, the less the relative dilution ratio of the substrate is. Overlapping ratio of 50% is appropriate for multi-pass laser cladding.