Structure, exchange interaction, magnetic properties and magnetostriction of series of cubic C15 MgCu2-type rare-earth transition-metal (R-T) compounds of Sm0.9Pr0.1(Fe 1-xAlx)2 (0⩽x⩽0.2) have been studied by X-ray diffraction, AC initial susceptibility, vibrating samples magnetometer and standard strain technology. The substitution of Al for Fe leads to an expansion of the lattice parameter α by 5.1%. Curie temperature Tc is reduced continuously by replacing Fe with Al, which is ascribed to the decrease of the R-T coupling strength. The intersublattice coupling constant JRT, molecular field coefficient nRT, and exchange field Bex , are evaluated by a molecular field model. The decrease of JRT with increasing Al content is related to the decrease of the number of (Sm,Pr)-Fe interacting pairs. Both the magnetostriction constant λ111 and the polycrystalline saturation magnetostriction λs at room temperature decrease with the Al addition, which can be ascribed to the decrease of the coupling strength.