The potential grain yield of maize increased tremendously from the 1980s to 2010s, as did the optimal plant density. However, cultivars were always bred in different environments owing to the varied eras in which they were created. To analyze the effect of genetic improvement on maize grain yield and related traits, we selected six hybrids from different eras for a detailed field study without water and fertilizer stress in 2017 and 2018 at Qitai Farm (89°34′E, 44°12′N), Xinjiang, China. The hybrids were selected based on the highest yields recorded in different years from 300 maize cultivars that had undergone 15 years of field testing. The maximum yield gap was approximately 20 % under the density with maximal yield measured from cultivar SC704 (the oldest cultivar) to DH618 and MC670 (the most recent cultivars) during the two experimental years. The global gain in yield potential was 559.3 kg ha⁻¹ year⁻¹ or 3.4 % with year of release (YOR⁻¹). The genetic gain in grain yield was 111.4 kg ha⁻¹ year⁻¹ (0.62 % YOR⁻¹) during the experimental years. The genetic contribution to potential grain yield was 18.0 %. Further analysis on the genetic progress of phenotypic traits showed that the leaf angle above ear decreased significantly by 0.26° year⁻¹ (1.1 % YOR⁻¹), which optimized the light distribution in the canopy and significantly improved the radiation use efficiency (RUE, 0.0286 g MJ⁻¹ year⁻¹ or 0.9 % YOR⁻¹). The ear ratio decreased significantly by 0.0034 year⁻¹ (0.71 % YOR⁻¹). Other morphological traits did not change significantly with the year of release, indicating that the breeding of maize cultivars from the 1980s to 2010s primarily focused on the leaf angle above ear and ear ratio in phenotype. For the physiological traits, the leaf area duration (LAD) increased by 0.4097 m²·d year⁻¹ (1.1 % YOR⁻¹), and the photosynthetic rate at maturity significantly increased by 0.268 μmol CO₂ m⁻² s⁻¹ year⁻¹ (3.7 % YOR⁻¹), which finally significantly increased the total biomass by 0.285 Mg ha⁻¹ year⁻¹ (0.96 % YOR⁻¹) and harvest index (HI) by 0.0021 year⁻¹ (0.45 % YOR⁻¹). Therefore, 18.0 % of the increased yield potential from the 1980s to 2010s was contributed by breeding under an optimal environment, among which the leaf angle above ear, ear ratio, LAD, photosynthetic capacity at maturity, biomass accumulation and HI played an important role for this genetic contribution.