Dual-fuel premixed charge compression ignition (DF-PCCI) combustion can achieve low nitrogen oxides (NOx) and particulate matter (PM) emissions for wide ranges of engine operations. However, the deterioration in thermal efficiency, and hydrocarbon (HC) and carbon monoxide (CO) emissions at low loads were recognized as the barriers for expanding the low-load operating range. In this study, the causes of the barriers were investigated and a mixture preparation strategy was suggested for overcoming the barriers in a natural gas (NG)-diesel DF-PCCI engine. Combustion and energy balance analysis was conducted to evaluate the strategy. Baseline DF-PCCI was determined by combinations of diesel start of injection (SOI) and NG substitution ratio (SR) at low loads from 0.3 to 0.6 MPa indicated mean effective pressure (IMEP). An increase in the homogeneity of a fuel-air mixture in the baseline DF-PCCI effectively reduced the NOx and PM emissions but increased the HC and CO emissions in each low-load operation. As the engine load was decreased, the formation of an overly-lean mixture intensified the effects of the mixture homogeneity. Therefore, the thermal efficiency, and HC and CO emissions deteriorated at 0.3 MPa IMEP. A mixture stratification strategy was established to increase the local equivalence ratio and reactivity of the fuel-air mixture. The strategy was realized by a retarded diesel SOI, a lower NG SR, and a higher exhaust gas recirculation rate. The strategy increased the degree of constant volume combustion by enhancing the combustion performance. The enhanced combustion reduced the combustion loss, and thus, improved the thermal efficiency. The HC and CO emissions also decreased mainly due to the improved combustion and the reduced mass flow rates of the NG.