In this paper, the effect of die-swell ratio on the microstructure and properties of as-spun fibers was systematically analyzed by electron microprobe analysis (EMPA) and the measurement of die-swell ratio, boiling-water shrinkage, porosity and mechanical properties. The formation mechanism of the spin orientation and skin-core structure were revealed. It was shown that with different die-swell ratio, the spinodal decomposition speed and the orientation degree between the outer and inner of as-spun fibers is different, which leads to a difference of denseness and hardness between the outer and inner, thus the skin-core and porous structures are formed. A small die-swell ratio leads to an enhanced spin orientation, a reduced skin-core difference and a more compact and homogeneous microstructure. As a result, the breaking tenacity of as-spun fibers and resultant precursors all increased.