Field experiment was carried out to study effects of ridge width and planting density on corn yields and water use efficiency in a plastic film mulching on ridge for rainwater-harvesting system. The system was designed by two widths of ridges of 30 cm and 60 cm that had three planting densities of high, middle and low. The results showed that soil water content and microbial biomass carbon content were increasing in the system. Corn height and biomass per plant decreased gradually but biomass per unit area increased gradually in the system with the increase of corn density under the same wide ridges. The rainwater- harvesting system with the ridge width of 60 cm could significantly increase chlorophyll content during a period of less precipitation, but significantly decreased the chlorophyll content in high corn density. The rainwater-harvesting system with the 30 cm wide ridge would make grain yield decrease due to the decreasing proportion of organic matter allocation to grain caused by intensive intraspecific competition. The rainwater- harvesting system with the ridge width of 60 cm and the high planting density had the highest yields and water use efficiency among the treatments.