The calculation of diffusion coefficient has a very important significance to the formation of the as-spun fiber[1].In our present work, Poly (amic acid) (PAA) fibers were prepared by the wet spinning way with a H2O/N, N-dimethylacetamide (DMAc) mixture served as the coagulation bath.The diffusion coefficient of DMAc was calculated by Nuclear Magnetic Resonance Spectroscopy (1H-NMR) based on the Crank equation[2] and the content of DMAc in the as-spun fibers was detected at the same time.