Marine magnetic lineation is an important media to interpret the age and spreading process of the oceanic crust. But there are many difficulties to identify the lineation from sea surface observation in a deep and narrow inactive ocean basin located in the low-latitude. Located in such a tectonic position, the spreading history of the Southwest subbasin of the South China Sea became a long time controversy. the deeptow data were able to increase observed anomaly amplitudes by a factor of ~4. However, there are many difficulties to distinguish the real magnetic lineation in this narrow low-latitude inactive ocean basin. We try some methods to improve the interpretation to data. Such as carefully corrected this bias by deskewing was explored to interpret it more accurately. Calibrated with IODP drill sites by recent IODP Expedition 349, the results show that the spreading ages of the Southwest subbasin is from 20 to ~15 Ma (from C6B to C5B). The full spreading rate range from 26 to 46 mm/yr, and its average is 37 mm/yr. We divided the profile into two sections along the ridge and summarize the common similarities and differences between the south and north slabs. It provides us a good chance to know that the spreading initiated with symmetric spreading but finally evolved into an asymmetric spreading basin with the rate difference reaching up to 10%.
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