Based on measured seabed sediment data and related hydrological and bathymetric data in northwestern Hangzhou Bay of China, the distribution characteristics of the seabed sediment have been studied and its influence on the seabed stability has been examined. The results are showed as follows: (1) Seabed sediment types in the deep trough are mainly sandy sediment with median grain size (d50) of 0.06–0.19 mm, and only a few clayey or silty sediments with d50 less 0.04 mm are distributed in the shallow waters. (2) Seabed sediments distribute with variation characteristics of fine ~ coarse from north to south, mostly with medium sorting degree. (3) Due to active seabed sediment with strong hydrodynamics, the local seabed experienced a seasonal changing characteristics.