The subbands occupation and subband transport properties in modulation-doped Al0.22Ga0.78N/GaN heterostructures are studied by means of magnetotransport measurements at low temperatures and high magnetic fields. The occupation of two subbands is observed from the Shubnikov-de Haas oscillations. It is found that the total density of the two-dimensional electron gas (2DEG) as a function of the electron sheet density in the second subband is linear. The threshold of the 2DEG density that the second subband begins to be occupied is 7.3×1012cm-2. The transport mobility of the 2DEG in the two subbands is obtained by using the mobility spectrum technique. It is found that the transport mobility in the first subband decreases significantly when the relaxation of the Al0.22Ga0.78N barrier occurs. The electron mobility in the second subband is much larger than that in the first one. The results indicate that the interface roughness scattering and the alloy disorder are the main mechanisms in determining the 2DEG mobility in AlxGa1-x N/GaN heterostructures.