In order to study the effect of solvent residuals on structure and properties of polyacrylonitrile fibers, the morphology, crystallinity and thermal properties of the fibers were discussed by Electron Microprobe Analysis, X-Ray Diffraction and DSC Analysis. It showed that the cross-section of the fibers with a lower solvent residual tends to be circular. The higher the solvent residuals are, the lower the crystallinity of the fibers and the bigger the crystal size. With the increase of solvent residual, the initial temperature of the fibers’ exothermic peak is in advance and the terminal temperature lags behind, the peak temperature increases, and the heat release also increases. Excessive solvent residuals make the heatrelease reaction very violent and the heatrelease increases. The ladder-shaped cyclation structure formed during the preoxidation process often breaks down because of the instant vast heatrelease.