Simiao rice, a high-quality indica variety widely cultivated in South China’s double rice cropping systems is vital for regional food security. However, the seasonal and varietal responses of its grain yield and quality to nitrogen (N) application remain insufficiently understood. This two-year (2022–2023) field experiment investigated three N application rates (N1: 105, N2: 135, N3: 165 kg ha−1) on multiple Simiao rice varieties, focusing on grain yield, milling, and appearance qualities. Higher N rates had no significant effect on early rice yield, but significantly increased late rice yield. The two-year average yield of N1 (5.71 Mg ha−1) was lower than N2 (6.23 Mg ha−1) and N3 (6.51 Mg ha−1) (P < 0.05), attributed to increased total spikelets. For early rice, the two-year average head rice rate increased with N (41.4% for N1, 42.6% for N2, 43.7% for N3), mainly driven by improved quality in high-chalkiness varieties (MXZ2, GXYLSM, HFYYX, and QXY19X). These varieties exhibited reduced chalky grain rate (26.0–20.8%) and chalkiness (10.0–7.2%) with increasing N, while low-chalkiness varieties (19X, TX812, XZXSM, and XXZ) showed no significant changes. For late rice, N rates did not affect milling or appearance quality. This study clarified that N application exerts distinct seasonal and varietal effects on dual-use Simiao rice: increasing N application significantly enhances late rice grain yield while improving early rice milling and appearance quality. These findings provide critical insights into tailored N management for high-quality and high-yield Simiao rice production in South China’s double cropping system.