In order to enhance the mechanical properties of polylactic acid (PLA) fiber, polypropylene(PP) was melt blended with PLA, then PLA/PP blend fiber was prepared by melt spinning. The thermal property, heat stability, tensile property and orientation degree of the PLA/PP fiber were tested using differential scanning calorimeter, thermal gravimetric analyzer, universal testing machine, fiber birefringence meter, respectively. The results show that the addition of PP has no significant effect on the glass transition temperature and melting temperature of PLA, but the crystallization behavior of PLA is improved, and the degree of crystallinity enhances by 585.9%. With the increasing of PP content, the heat stability of PLA is decreased, especially the initial decomposition temperature is decreased obviously, but the carbon yield is increased. Meanwhile , the degree of orientation and mechanical properties of the PLA/PP fiber are enhanced. When the PP mass fraction is 20%, the orientation degree, breaking strength and elongation at break of the PLA/PP fiber increase 55.6%, 98.2% and 44.4%, respectively.