The study is based on the acoustic data collected by horizontal line array (HLA) of Asian Seas International Acoustic Experiment (ASIAEX) in South China Sea of 2001. Beamforming was used to localize the sound sources during the experiment, such as explosive, to understand its correlation with position deviation from the array. According to previous studies, the horizontal line array has ambiguity in bearing identification, which was often resolved by maneuvering array¡¦s position, therefore it can also be concluded that the well understanding of array¡¦s position is the key to the accurate source localization. Due to the limitation of element spacing of array used in the experiment, 50 Hz is the highest frequency can be analyzed in the study, so the recorded explosive sounds were chosen for analysis. The numerical test of array has shown the width of main lobe in beamforming is reduced by the increasing total length of the array, and the energy in the side lobes would affect the accuracy of source bearing. During the experiment, the horizontal array was found, and proved indirectly by current measurement of acoustic Dopper current profiler, to be moved by tidal currents, so the deviation from the designed deploy position can be computed. The deviation was used to quantify the resolution effect on localization by the movement of array, and when there is a 10 m deviation, the width of main lobe would increase from 8¢X to 16¢X in the studied case. The experimental results match the simulations well, so it can provide a guideline in predicting the accuracy of underwater source localization when the movement of horizontal line array is possible. Key word: underwater source localization, horizontal line array, beamforming, position deviation, array resolution