One of the most important applications of acoustic well logging is to evaluate the cementation quality in the casing hole; such evaluation is in fact a physical question of detecting the position of the fluid-filled strip channel that is actually distributed in a limited azimuthal and axial zone behind the casing pipe. Currently, a common detection method is to measure the amplitude of the casing wave (ACW) in different azimuths and then reverse the position of the strip channel. Thus, in this paper, a 3D finite-difference algorithm is developed to study the azimuthal distribution of ACW caused by the strip channel. The simulation results show that ACW is affected by many parameters such as the radial position of azimuthal receivers and the azimuthal angle of the strip channel. More importantly, the product of the inner diameter of the casing pipe and the central frequency of the source will determine the azimuthal distribution of ACW: ACW reaches the maximum at the opposite azimuth of the strip channel when the product is below the threshold value, and it reaches the maximum at the central azimuth of the strip channel when the product is above the threshold value.