This study was conducted to determine the effects of plastic film mulch on percentage of abortive tillers and harvest index (HI) in two spring wheat populations, and to explore its mechanism in relation to size inequality and life-history strategies theory. Field-grown spring wheat mulched with plastic film shows a remarkable increase in grain yield (an average increase of 38·5% over non-mulched control), which mainly results from the significant increase in above-ground biomass (+44·7%). At the same time, however, reproductive allocation (spike weight/above-ground biomass, −5·2%) and harvest index (−4·5%) were significantly lower (P<0·05) in mulched than unmulched populations in both of the two cultivars used. Meanwhile, the number and weight of barren shoots and the ratio of barren shoot biomass to total shoot biomass were significantly greater in mulched populations than unmulched controls both at booting, flowering and ripening stages. From tillering to ripening stages, the tendency of G (Gini coefficient) shows obvious differences between mulched populations (‘∧’ type with the peak of G at booting stages) and non-mulched controls (‘∨’ type with the lowest G at flowering and rebounded at ripening stage). At booting and flowering, the G was significantly higher in mulched populations than unmulched controls, and it was just contrary at ripening. Reproductive allocation and HI were both negatively correlated to average G of the populations, which indicated that resource use efficiency of a crop is inversely related to plant-to-plant variability and also provided further evidence for the views that stand uniformity of field crops is an important aspect of high yield formation. From these results, it was concluded that appreciable growth redundancy occurred in spring wheat populations mulched with plastic film, which may result from the exacerbated inter-plant competition and self-thinning (evidenced by the increased degree of Gini coefficient at booting and flowering stages). Thus, spring wheat cultivation with plastic film mulching does not always mean efficiency, although there is a remarkable increase in grain yields.