Ultrathin g-C3N4/Al2O3 hybrids are prepared via in-situ reaction. The structure modification role of in-situ formed HNO3 for g-C3N4 is found. The ultrathin g-C3N4 nanosheets are formed by the acidified melamine and Al(OH)3. In-situ calcination of melamine and Al(OH)3 benefits the contact of C3N4 and Al2O3. The activity of g-C3N4/Al2O3 is 16.6 times that of pristine g-C3N4 in degrading RhB.