Abstract: The main analytical method of unsteady flow in pipelines includes frequency domain analysis and time domain analysis. The existence of the non-linear friction makes the analysis difficult. Usually, the pipeline friction in unsteady flow is regarded as to be equal to that under the steady-state flow. But for long period dynamic simulation of unsteady flow in liquid pipelines, using the conventional "quasi-stationary" hypothesis to calculate the unsteady headless was insufficient. The frequency-dependent friction model can change this situation. Presented in this article is the formula of frequency-dependent friction and its use is MOC (Method of Characteristics).