Mixed sediments mean the mixed sedimentary carbonate clast and siliceous clast, which are common in modern and ancient sediments. Based on the new seismic data, elementary study is finished to mixed sediments in west deepwater area, Northern South China Sea. The mixed sediments have some reflection features such as parallel-subparallel, medium-strong amplitudes, medium frequency and medium continuity on seismic profiles. The according area of mixed sediments that lies in the northwest of seismic area develop in a relatively wide range with an area of 1,084km2 and up to 138m of vertical thickness in middle Miocene Meishan Formation. A model of mixed sediments is set up based on the tectonic evaluation and seismic exploration in northern South China Sea. These mixed sediments that lie in the semi-deepwater area derive from carbonate clast and parts of siliceous clast, which are provided by Xisha-Guangle Terrace on the east-southeast, and siliceous clast, which are supplied by Indochina peninsula on the west. The main factors controlling the mixed sediments are tectonic action (including changes of sea levels), source supply and hydrodynamic condition, which are closely relative each other.