It's highly desired to design and fabricate effective Z-scheme photocatalysts by promoting the charge transfer and separation. Herein, we firstly fabricated the ratio-optimized g-C3N4/alpha-Fe2O3 nanocomposites by adjusting the mass ratio between two components through a simple wet-chemical process. The resulting nanocomposites display much high photocatalytic activities for CO2 conversion and phenol degradation compared to bare alpha-Fe2O3 and g-C3N4. Noteworthily, the photocatalytic activities are further improved by constructing Al-O bridges, by 4-time enhancement compared to those of alpha-Fe2O3. Based on the steady-state surface photovoltage spectra, transient-state surface photovoltage responses, photoelectrochemical I-t curves and the evaluation of produced center dot OH amounts, the exceptional photoactivities of Al-O bridged g-C3N4/alpha-Fe2O3 nanocomposites are attributed to the significantly promoted charge transfer and separation by constructing the g-C3N4/alpha-Fe2O3 heterojunctions and the Al-O bridges. Moreover, the charge transfer and separation of this photocatalyst have been confirmed to obey the Z-scheme mechanism, as supported by the single-wavelength photocurrent action spectra and single-wavelength photoactivities for CO2 conversion. Furthermore, the mechanism of the photocatalytic CO2 conversion has been elaborately elucidated through the electrochemical reduction and the photocatalytic experiments especially with isotope (CO2)-C-13 and D2O, that the produced H atoms as intermediate radicals would dominantly induce the conversion of CO2 to CO and CH4.