According to the heat exchange theory, a new calculating method of μ value in the unsteady heat transfer equation under the convective boundary condition was inferred, which was compared with the first μ value of plane wall convection that solved by the transcendental equation. When the value is from 0.01 to 0.6, the result shows that the μ value becomes bigger and bigger than the first value solved by transcendental equation solution. When the value is from 0.6 to 100, the values of the two methods are moving close to and overlapping. Using the new method to calculate the μ value of the heating process of 10.8t cold and hot steel ingot, the calculated result is close to the experimental result.