Red algae represents an important marine bioresource. One high-value utilization of red algae is the production of agarooligosaccharides which have many positive biological effects. However, the lack of an efficient production route seriously limits the application of agaro-oligosaccharides. In this study, we established a green route that combines chemical liquefaction and enzymatic catalysis for the efficient production of agaro-oligosaccharides, and we used the production of neoagarotetraose (NA4) as an example. Agarose (150 g L −1) liquefaction by citric acid was controlled with respect to two targets: a 100% liquefaction rate and a high average degree of polymerization (> 4) of the liquesced agaro-oligosaccharides, which were then catalyzed by β-agarase into an oligosaccharides mixture with a high concentration of NA4 (30.8 g L −1) in a 1-L reaction volume. After purification, we obtained 25.5 g of NA4 with a purity of 92%. This work establishes an easy route for the efficient production of pure agaro-oligosaccharides from agarose.