The Bohai Sea is an important distribution zone for the temperate population of finless porpoises. However, very few reports has been published on the finless porpoise population in the region. Six surveys were conducted between May to July 2015 and in September and November of 2016. They were based on the line-transect methods in the Bohai Sea. These surveys were meant to explore population status of the finless porpoise in Bohai Sea, which could provide basic data to develop ecological protection strategies for the finless porpoise in the sea. The total effective trackline length of six surveys was 8972 km. A total of 136 individuals and 87 groups of finless porpoises were witnessed. More finless porpoises were observed near the Yellow River estuary, to the northeast of the Qinhuangdao and in the coast of Laizhou Bay. Of the witnessed groups, sightings occurred more at between 14:00–18:00, in the area 15–35 km offshore. Spatially, the finless porpoise can be found in most of the Bohai Sea, including the three bays and central waters. Of the seasons surveyed, the finless porpoise groups were observed frequently in the waters 20–40 km out of the Yellow River estuary in May. The probability values of group sightings on the trackline were 0.399, 0.085 and 1 respectively in Distance 7.0 software, The finless porpoise population number in the whole Bohai Sea was estimated to number 7883(CV=0.21), 3701(CV=0.21), and 3124(CV=0.21) individuals, and its population density was estimated to be 1.441, 0.066, and 0.056 individuals/km² correspondingly. Compared with those in the southern Yellow Sea and the Inland Sea of Japan, the population density of finless porpoise in the Bohai Sea was relatively smaller. Limited by uncertain effects from the weather, sea conditions and observers’ experience, the estimated values given above need to be confirmed in the future. However, despite the current deficient state of the data, the size of the population of finless porpoise in the Bohai Sea was here estimated for the first time and this work could serve as a reference for future study.