The electronic structures, elastic constants and Mullikan population of Nb and Al codoped MoSi2 were calculated by the density functional theory. The results show that the co-doping of Nb, Al can improve greatly the toughness and conductivity of MoSi2. When the doping volume of Nb and Al reaches to x= 0.25, the elastic modulus of MoSi2 decreases from 408.86 GPa to 261.30 GPa. By analyzing the density of states and Mullikan population, after doping the total density of states has a blue-shift, the charge density on Fermi level increases, the conductivity of the system improves and Mo-Al and Nb-Si bond populations decreases with their bond lengths increase. In addition, the covalent between atoms weakens, while the covalent bonds between Al and Si enhances. As the distribution of bond length and bond energy along each crystal orientation tends to be symmetrical, the fracture toughness of doped MoSi2 would improve.