Full-potential linearized augmented plane-wave (FP-LAPW) method within the local density approximation plus self-interaction correction (LDA+USIC) has been applied to study the structure stability and electronic structure of ReSi1.75 and its doped systems with Al and Mo. Structural relaxation results show that the vacancy prefers to occupy the Si3 and Si4 site in the lattice with little ordering. For doping systems, Al prefers to substitute for Si at the Si3 site and Mo prefers to substitute for Re at the Re1 site. ReSi1.75 shows narrow gap semiconductor behavior with an indirect gap of 0.12 eV and a direct gap of 0.36 eV. Al doping compound remains semiconductor while Mo doping compound has a tendency to change into semimetals or metals. The Fermi level of doped systems moves into the valence band resulting in an increase of density of state at the Fermi level. It will enhance the thermoelectric properties and agrees well with the experiment results.