Using sediment strata and the abnormity of gravity and magnetism, the spreading age and character of northwest sub-sea basement of South China Sea was discussed. The sea floor spreading age was deduced at early Oligocene at the beginning and at the forepart of late Oligocene in the end. Twice periods of sea-floor spreading were implicated by stratum deformation, passive overlap, oceanic crust configuration and its symmetry. The first one occurred at early Oligocene during which the upper Eocene was snapped, deformed and overturned to spreading center on account of sea floor expanding, subsiding, and its pushing and shoving. The second one took place at the forepart of late Oligocene during which lower Oligocene was pulled apart at a limited distance because of short time and lesser range of sea-floor expanding. As a result of the different degree of lithosphere thinning and extending, the northwest sub-sea basement show a triangle with a wide edge in northeast and becoming narrow and vanishing to the southwest. The ocean crust boundary was deduced at the turning of the northwest sub-sea basement in southwest region and at turning of the northwest sub-sea floor in northwest region based upon the upper Eocene snapped points and the abnormity of gravity and magnetism of northwest sub-sea basement. In addition, the abnormity of gravity and magnetism indicate that rudimental continental crust may exist in the neonatal ocean crust. These phenomena, mentioned above, illuminate that the northwest sub-sea of South China Sea came to an untimely end when the sea floor spreading was at its bud period.