Polyactic fibers have superior biodegradability, moisture absorption, resiliency and processibility and can be used in various fields especially in blood filtration. Polyactic can be melt blown into nonwoven fabrics. To predict the fiber diameter of the polyactic melt blown nonwoven fabric, the air drawing model of polyactic was established. The predicted fiber diameter tallies well with the measured fiber diameter. Computer simulations of the effects of the processing parameters on the fiber diameter were performed with the help of the air drawing model. The simulation results show that smaller polymer flow rate, larger initial air velocity and larger die-to-collector distance can all produce finer fibers while too large initial air velocity and too large die-to-collector distance contribute little to the air drawing of polyactic melts.