Using the recent high-resolution seismic data of three years, the author analysed the seismic facies combination and inner feature of turbidites in detail and discussed the change of sea level combining turbidites’ characteristics evolution of different epochs and different layers since late Miocene in the northwest sub-basin of the South China Sea (SCS). The turbidites exhibit with channel-levee complex during late Miocene to Pliocene and with lobe facies that channel facies is not distinguished during Quaternary. It is the prominent trait of turbidity action in the northwest sub-basin of the SCS since late Miocene that channels incised continually and retreats from abyssal plain towards the slope. The channel evolving process displays an ascending trend as a whole of sea level since late Miocene. The turbidites seismic facies’ change in the inner sequence reveals the sea level went through respectively not less than two ascending-descending cycles during late Miocene and Pliocene and sea level is early ascending stage after descending quickly during Quaternary. The sea level’s changing trend in the northwest sub-basin of SCS is coincident basically with global Eustacy since late Miocene.