Double perovskite Sr2FeMoO6 with Al doping at Fe site up to 30% were prepared by traditional solid-state reaction. The degree of the cation ordering of Sr2FeMoO6 increases with Al doping obviously as shown by the refinement of room-temperature X-ray diffraction. The saturated magnetic moment per formula unit of these compounds decreases gradually with increasing Al concentration, but the magnetic moment per Fe ion increases markedly, which is consistent with the improvement of cation ordering. Basing on the analysis of magnetic structure with Al doping, doping of Al will lead to the formation of non-magnetic coupled Mo—O—Al—O—Mo chains, which can divide the large Mo—O—Fe ferrimagnetic region into smaller patches and enhance the low field magnetoresistance. The formation of Fe—O—Fe antiphase boundaries are suppressed by the doping Al due to the improvement of the cation ordering, and the high field magnetoresistance that roots in spin-dependent scattering at the antiphase boundaries is reduced. Therefore, the total magnetoresistance reaches a maximum value at x=0.15.