Ridge–furrow mulching with plastic film is widespread in northern China because it can significantly improve crop yields. Whether maize grain yield would further increase with higher plant density, due to the better soil moisture conditions under plastic mulch, is unknown. A process-based model (DeNitrification–DeComposition) coupled with a field experiment during the 2014–2015 growing seasons was used to evaluate the effect of plant density on yield. The field experiment included three treatment groups: soil fully covered with transparent polyethylene film (PM) with plant densities 35,000 (PM 35 ), 45,000 (PM 45 ), and 65,000 plants ha –1 (PM 65 ) and bare soil with a maize planting density of 45,000 plants ha –1 as the control (CK). The DNDC simulated yield in PM and CK was well-matched with the measured values and maize growth pattern, and explained 64% of the yield variation and had an RMSE of 176 kg ha –1 . At the same planting density (45,000 plants ha –1 ), individual plant weight, grain weight per plant, 100-seed weight, and grain yield in the non-mulched plots were 45, 28, 52, and 28% of the values for mulched plots, respectively. In the mulched plots, the individual plant size, grain weight, and seed size significantly decreased as planting density increased. The PM 65 had 8% and 19% higher grain yield than PM 45 and PM 35 , respectively. Increasing planting density above 45,000 plant ha –1 in the PM resulted in a small but non-significant increase in grain yield. We concluded that increasing plant density in PM will not necessarily increase maize yield, and the optimum planting density could be determined by combining the field experiment with the model.