Southwest sub- basin (SWSB) in the South China Sea (SCS) is “V” shaped with unfold curving towards northeast and pointing to southwest. Fully understanding the opening kinematics of the SWSB is crucial to study the tectonic evolution of the SCS in Cenozoic. Data of reflection seismic profiles, gravity and magnetic anomalies, chrono- stratigraphy from IODP 349 are used to study the geological and tectonic characteristics in this paper.Results showed that the features of the onlap sequences at the northeastern and southwestern edges are almost the same, which reveals these sequences were deposited in the same period of sea- floor opening, and the trend of the Zhongnan- Liyue fault zone as the boundary between SWSB and East sub- basin (ESB) is not completely straight from north to south but curves to west on both tails, resulting in an arc- fault zone, resulting from the fanshaped migration of sea- floor spreading of the SWSB. At western side of SWSB, there is another fault zone with its fault- face tilting to west, separating oceanic crust from transitional crust areas. A set of sedimentary sequence cover both oceanic crust area and transitional crust area with Paleogene sediment formed during the extensive stage, which exhibits the synchronous extension of the two areas. All the geological and tectonic appearances mentioned above verified that the sea- floor spreading pattern of SWSB had the scissors type opening. This basin opened from northeast to southwest at the same time but the rate of spreading was very slow, forming a large area of sea- basin with oceanic crust at the east and a small area with transitional crust at the west in the SWSB. Thus, the plane “V” shape of SWSB has been taken shape.