Abstract: With the implementation of the Development Plan of Shandong Peninsula Blue Economic Zone, human activities in Longkou Bay have developed at an unprecedented rate. Economic development not only affected its tidal and sedimentary environment, but also made petroleum one of the major pollutants in the bay. The surface sediments (0-5 cm) were collected in Longkou Bay. All samples were analyzed for hydrocarbons using GC-MS. The distribution and characteristic parameters of n-alkanes and polycyclic aromatic hydrocarbons (PAHs) were investigated to determine the origins of organic matter and pollutant emission in sediments. Then an environmental risk assessment of sediments were carried out. The results showed that the maximum value of CPI_25-35 was 1.5 (< 4). The TAR ranged from 0.15 to 1.08, while the P_mar-aq ranged from 0.28 to 0.74. These indicated that biogenic origin n-alkanes were more input of marine plankton. The maximum value of the nC₁₆ index was 20.76 (< 30), and 71% of the stations' NAR value was less than 0. These showed that marine development activities caused a certain degree of oil pollution. The proportion of 2- and 3- ring PAHs reached up to 50%. The total concentration of 16 PAHs ranged from 460.03 to 2448.29 ng/g, with a mean value of 1328.60 ng/g, and 86% of the stations reached the moderate and severe pollution. The indices of PAHs indicated that PAHs were derived primarily from petroleum and combustion. PAHs toxicity level was evaluated using the Canada Soil Quality Guidelines (SQGs). Some low molecular weight PAHs (Acy, Ace, Flu, Phe, and Ant) occasionally had adverse ecological risk. The Carcinogenic PAHs (∑Car-PAHs) and BaP toxicity equivalent (TEQ_BaP) were used to estimate the potential health risk of PAHs. They indicated that Longkou Bay sediments had certain toxicity risk, but they were within safe levels. This study shows that n-alkanes in Longkou Bay sediments come from terrestrial, marine organic matter and hydrocarbons origin. And PAHs belong to a mixed source of petroleum and combustion, and their environmental risks are at safe levels.