Pulsed infrared thermal wave testing, which typically uses reflective-type incentives, is a new non-destructive testing technology. In view of the reflection method with big defect depth quantitative detection error, transmission infrared thermal wave method was systematically analyzed in order to improve the detection precision. The principle of infrared measurement of defect depth was discussed by analyzing the material in pulsed thermal excitations of one-dimensional heat transfer model. The relationship between the characteristics of time and defect depth was established by the peak time method of the first order differential of surface temperature, so the quantitative detection on the depth of defect was realized. The sample of PVC flat with artificial slot wedge defects was used. The depth of defects were detected by comparative experiments between transmission and reflection methods, and the measurement errors were analyzed. The results show that reflection method in quantitative calculation for defects requires selecting reference region, while transmission method does not rely on the reference avoiding the error resulted from it. Moreover, transmission method directly heats surface defects, so short response time can be obtained and the defect depth precision are improved greatly by solving the characteristic time of the defect.