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Delays in sowing have significant effects on the grain yield, yield components, and grain protein concentrations of winter wheat. However, little is known about how delayed sowing affects these characteristics at different positions in the wheat spikes. In this study, the effects of sowing date were investigated in a winter wheat cultivar, Shannong 30, which was sown in 2019 and 2020 on October 8 (normal sowing) and October 22 (late sowing) under field conditions. Delayed sowing increased the partitioning of 13C-assimilates to spikes, particularly to florets at the apical section of a spike and those occupying distal positions on the same spikelet. Consequently, the increase in grain number was the greatest for the apical sections, followed by the basal and central sections. No significant differences were observed between sowing dates in the superior grain number in the basal and central sections, while the number in apical sections was significantly different. The number of inferior grains in each section also increased substantially in response to delayed sowing. The average grain weights in all sections remained unchanged under delayed sowing because there were parallel increases in grain number and 13C-assimilate partitioning to grains at specific positions in the spikes. Increases in grain number m–2 resulted in reduced grain protein concentrations as the limited nitrogen supply was diluted into more grains. Delayed sowing caused the greatest reduction in grain protein concentration in the basal sections, followed by the central and apical sections. No significant differences in the reduction of the grain protein concentration were observed between the inferior and superior grains under delayed sowing. In conclusion, a 2-week delay in sowing improved grain yield through increased grain number per spike, which originated principally from an increased grain number in the apical sections of spikes and in distal positions on the same spikelet. However, grain protein concentrations declined in each section because of the increased grain number and reduced N uptake.