Low-temperature two-step concentrated H₂SO₄ was discovered to be a solvent with high cellulose solubility [>300 g/L (17 wt%)], fast cellulose dissolution, high regeneration yield (>0.92 g/g), and cellulose being mouldable during regeneration. The superior performance was enabled by the much better compatibility of cellulose with lower concentrated H₂SO₄ at low temperature, compared with that of high concentrated H₂SO₄. The regenerated cellulose was characterized by mostly unchanged composition and highly tunable degree of polymerization (DP). The H₂SO₄ starting content, cotton fibre temperature, dissolution temperature, regeneration temperature, regeneration bath and storage time were factors determining the DP of regenerated cellulose, which could be equivalent to 4–90 % of the original cotton. These advantages of the solvent enabled versatile application in fabrication of extra strong cellulose hydrogels, manufacture of strong cellulose fibres, preparation of various homogenous composites which would be prepared with much more difficulty by using other solvents, and facile manufacture of cellooligosaccharides.