The performance of matched-field localization with a horizontal line array (HLA) at different depths in shallow water is investigated by normal mode modeling and computer simulation. It is observed that the matched-field localization performance of a bottom-mounted HLA is better than that of a HLA at other depths in shallow water. The normal mode modeling is used to explain this result. According to the normal mode theory, the mode shape is a function of water depth. The absolute values of the modes at different depths are compared. It is shown that the number of modes whose absolute values close to zero is smaller at the bottom than at other depths. It means that the HLA deployed at the bottom can sample more modes，obtain more information of the source and achieve better localization performance.