In this paper, considerable magnitude of spin splitting for the conduction subband at the Fermi energy is obtained in AlGaN quantum wells (QWs) grown along the c-axis. We have analyzed how the magnitude of spin splitting of the first electron subband in AlGaN QWs with different sheet carrier concentration changes as a function of applied gate voltage, well width, and Al content in the barrier. It is also found that the contribution to spin splitting from Dresselhaus term is much larger than that from Rashba term, the contribution of Dresselhaus term to the total spin splitting depends greatly on the carrier concentrations, the change of well width has little effect on total spin splitting, and the magnitude of spin splitting can be greatly modulated by Al content in the barrier, gate voltage, and sheet carrier concentration. The internal polarized electric field is crucial for considerable spin splitting in III-nitride QWs. Moreover, the magnitude of total spin splitting calculated here is comparable with other theoretical and experimental values observed in III-nitride heterostructures.