Abstract: In this paper,we analyzed the characteristics of attitude errors caused by imperfect alignment of the MRU with the reference frame of vessel,and influences on the positions of beams in multibeam survey in detail.In this case,according to the rules that depth error is mainly determined by roll error,and roll error is mainly related with pitch,the 3D sun-illuminated terrain of seafloor,the math tool of Fourier or Wavelet,and the side projection of depth errors are used to confirm whether imperfect alignment of the MRU exists or not.If it exists in multibeam survey,we can obtain the offset angle when the linear relations of pitch and depth errors are disappeared.Finally,all data which are impacted by the imperfect alignment of the MRU will be corrected by the offset angle.It provides a reference for improving multibeam bathymetry,especially in shallow water.