Kramers-Krnig (K-K) relationship is widely used in diagnostics of longitudinal bunch distribution with frequency domain measurement, and it has been used as a powerful tool in analysis of bunch profile and length. The study results show that the bunching beam parameters are severely affected by the choosing of the baseline of autocorrelation curve, power loss at low frequency and cutoff at high frequency, as well as extrapolation point, etc. As an example, the longitudinal bunching length is measured by means of coherent transition radiation in accelerator laboratory of Tsinghua University. We analyze the influence of the choice of parameters on measured experiment results and discuss the method of choosing the key parameters for the K-K transform.