When plane waves propagate through a medium flowing in a pipe, the general relation between the pressures at three adjacent points equally spaced has been derived. And a new method is proposed to measure the mean flow velocity in a pipe from multiple measurements of acoustic pressure using microphone array. It is based on the fact that the variation of mean flow velocity affects the change of wave number. There are some advantages of this approach such that it does not obstruct the flow field and can provide the time-spatial mean flow velocity. Some numerical simulations and experiments are accomplished to verify the utility of this method.