The seasonal and diurnal changes of horizontal and vertical spatial distribution of fish were studied by applying the hydroacoustic method in Lake Tianmu. The results showed that there was regular horizontal migration of fish between different seasons and no significant difference in horizontal distribution between day and night in Lake Tianmu. However, there existed a slight diurnal change in the inshore-offshore horizontal migration in some parts of the lake area during the late spring and in the summer. Significant differences in vertical distribution were found between seasons and between day and night. The vertical distribution of fish density was significantly affected by the water temperature stratification in July when there was a thermocline, and the fish density in hypoliminion was almost zero. The fish in Lake Tianmu was distributed in groups in the four seasons, but the typical cluster form occurred only in January. Relative to the daytime, the fish distribution at night was more uniform and scattered. Moreover, horizontal survey showed that a large number of fish was spread in the upper water layer at night.