Carbon dioxide reduction (CRR) is an attractive strategy for alleviating global warming and producing valuable fuels. In this work, we study the catalytic conversion of CO2 to C1-C3 products on boron nanosheet in the presence of compressive strain by using density functional theory. Thermodynamic and microkinetic models are applied to demonstrate the favorable products, critical steps, and hydrogenation mechanisms. As demonstrated, the strain can turn metallic two-dimensional boron nanosheet to semiconductor, not only making boron sheets possess photo(electro)catalytic activity, but also improving energy efficiency and selectivity performance against hydrogen evolution reaction. By introducing the aqueous electrolytes, the hydrated alkali cations not only effect on the CO2 concentration, but also produce a bigger surface charge density and stronger interfacial electric filed. Especially, the selectivity of C2+ products is enhanced with the increase of alkali cations size by decreasing the kinetic barrier for CO dimerization and stabilizing the intermediates. The results highlight the significance of metal-free catalysts for CRR by the photoelectrochemical method and provide novel avenues for the development of new solar-energy utilization catalysts.