In this study we analyze one of a CFD for timing discrimination. Walk error, drift and precision are the three performance parameters of timing discrimination. The walk error is the most important error type generally. Firstly, we divided the waveform into two types. One is the Gaussian waveform distribution which has three parameters: amplitude, mean, and the pulse width; and the other is Rayleigh waveform distribution which has two parameters: mean and pulse. We analyzed different situations with their changing parameter, and the drift value of time can be obtained for each parameter changing.