Abstract: The East Asian finless porpoise (EAFP, Neophocaena asiaeorientalis sunameri) is a Grade II nationally protected species in China, and classified as Endangered in the International Union for Conservation of Nature Red List of Threatened Species. Supporting effective conservation efforts for this species by updating current ecological information, such as group size and distribution patterns, in key habitats, such as the Miaodao Archipelago, is therefore important. In the present study, data on group size and spatial distribution of EAFPs in summer were collected using passive acoustic technology within Miaodao Archipelago waters. Finless porpoises were most often observed acoustically in small group size (93.55% were single or in pair) and heterogeneous distribution patterns. Generally, acoustic detection rates decreased from west to east, while the highest rates were found in the western areas of the Tuoji, Daqin and Huangcheng Islands, where human disturbance level is low. These findings improve our basic ecological understanding of the EAFPs in Miaodao Archipelago in recent years. Specifically, these new data offer insights into the distribution pattern, habitat selection, and potential impact factors of the porpoises in Bohai and Yellow Seas.