In the clostridial acetone-butanol-ethanol (ABE) fermentation, the intermediate acetate and butyrate are re-assimilated for solvent production. Here, key genes in ABE pathways in Clostridium saccharoperbutylacetonicum N1-4 were overexpressed to enhance acid re-assimilation and solvent production. With the overexpression of sol operon, acid re-assimilation was enhanced, and ABE production was increased by 20%, with ethanol production increased by six times but almost no increase in butanol production. To further drive carbon flux for C4 metabolites and ultimate butanol production, key genes including hbd, thl, crt and bcd in butanol production pathway were further overexpressed. Compared to the control, butanol, acetone and total ABE production in the new strain was increased by 8%, 18%, and 12.4%, respectively. Finally, simultaneous saccharification and fermentation was carried out using acetate-pretreated switchgrass. 15.4 g/L total ABE (with a yield of 0.31 g/g) was produced in both engineered strains, which was significantly higher than the control.