A simple method for obtaining pure and well-defined oligosaccharides was established by hydrolyzing agar with β-agarase from Vibrio natriegens. The conditions for enzymolysis were optimized as follows: a temperature of 45 °C, a pH of 8.5, a substrate concentration of 0.3%, an enzyme amount of 100 U/g and an enzymolysis time of 20 h. Neoagaro-oligosaccharides with different degrees of polymerization were obtained by hydrolyzing agar with β-agarase for different lengths of time. After removing pigments using activated carbon and salts by dialyzing, the enzyme hydrolysis solution was separated with Bio-Gel P2 column chromatography. Neoagaro-oligosaccharides with different degrees of polymerization were acquired. By comparing with authentic standard substances, along with further confirmation by FTIR, MS and NMR, structures of the purified neoagaro-oligosaccharides were identified as neoagarobiose (NA2), neoagaroteraose (NA4), neoagarohexaose (NA6), neoagarooctaose (NA8), neoagaro-decaose (NA10) and neoagarododecaose (NA12) with purities of more than 97.0%. The present study established a method for the preparation of various neoagaro-oligosaccharides that may be of great significance for further study of their bioactivities.