Low planting density and lack of density-tolerant varieties are the critical factors limiting the yield of sweet maize in Southeast China. However, there is relatively limited information about the interaction effects of plant density and variety on sweet maize in Southeast China. A two-year (2021–2022) field experiment was conducted with two sweet maize varieties (MT6855 and XMT10) and three plant densities (D1: 45,000 plants ha−1, D2: 60,000 plants ha−1, and D3: 75,000 plants ha−1). The results showed that plant density and variety had significant interaction effects on sweet maize yield and most traits. Increasing plant density significantly increased the fresh ear yield of MT6855, while it did not affect the XMT10 variety. The increase in fresh ear yield for MT6855 under D2 treatment was 14.9% and 14.0% higher than that under D1 treatment in 2021 and 2022, respectively. Meanwhile, increasing plant density significantly increased the bare plant rate and decreased the number of grains per ear of XMT10, while no sustainable changes were observed in MT6855. Moreover, we observed significantly increased pre-silking dry matter, N, P, and K accumulation across different densities in both varieties. In contrast, during the post-silking stage, the increasing plant density significantly improved the accumulation of dry matter, N, P, and K, only in MT6855 but not in XMT10. Meanwhile, harvest index, dry matter remobilization, and leaf N, P, and K remobilization of MT6855 were significantly higher than those of XMT10. Increasing plant density significantly increased N, P, and K partial factor productivity of MT6855 but did not remarkably affect those of XMT10. In addition, fresh ear yield of sweet maize was significantly and positively correlated with pre-silking, post-silking, and total N and P accumulation but had no significant relationship with leaf K remobilization across the two varieties. These results suggest that MT6855 is a density-tolerant variety of sweet maize, and optimizing planting density with a density-tolerant variety can improve the accumulation and remobilization of dry matter and nutrients, thereby improving the fresh ear yield and nutrient use efficiency of sweet maize.