Abstract: Correlation Sonar obtains the velocity of vessel by least-square fitting the theoretical correlation function and data correlation function or other optimization method, so the veracity of theoretical model is very important for velocity calculation. After massive data processing, it was found that, under some circumstances, the shape of the theoretical correlation function and data correlation function in three-dimensional space doesn't match well, which leads to less accurate of the velocity measuring results. By adapting the horizontal projection of the model from circle to ellipse, the theoretical correlation function is improved to fit the data correlation function more precisely, the velocity measuring accuracy improves accordingly. The conclusion that the ellipse shape of the correlation function is caused by the inhomogeneity of the sea-bottom interface roughness is obtained.