An unsteady numerical model for double-diffusive natural convection of low Prandtl number liquids with Soret and Dufour effects inside the horizontal cavity is developed. The thermosolutal model is solved numerically using the SIMPLE algorithm with QUICK scheme. The flow field, temperature and concentration distributions for different buoyancy ratios, Rayleigh numbers and aspect ratios under different Prandtl numbers are studied systematically. The results reveal that the flow structure develops from conduction-dominated to convection as buoyancy ratio increases under different Prandtl numbers. Heat transfer intensity keeps constant and mass transfer intensity grows slowly before a critical point as Rayleigh number increases for different Prandtl numbers. Then heat transfer intensity enhances dramatically and mass transfer intensity increases faster as Rayleigh number increases after a critical point. The vortex number of flow structure, recirculation zones of isotherm and isoconcentration contours, heat and mass transfer intensity increase as aspect ratio decreases under different Prandtl numbers.