Band gap renormalization of ZnO/Mg0.1Zn0.9O single quantum well (QW) with gradual well width (Lw) is studied by room-temperature time integrated photoluminescence (PL) spectra at high excitation power density. The photo-generated carrier density is n=1.6×1014 cm-2 and the magnitude of red shift of PL spectrum peak increases from 5.9 meV to 97.1 meV with Lw changing from 2.3 nm to 4.3 nm. With Lw increaseing, the red shift increases but the increase rate gradually decreases. When Lw>2αB(αB, the exciton Bohr radius of ZnO bulk, is about 2 nm)， the red shift starts to be gradually saturated. It is found that the red shift is the competition result of energy gap contraction due to many body effect and intraband filling effect at high excitation power density. The result is useful for designing and application of ZnO QW-based optoelectronic devices.