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. 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. 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.