Abstract: Abstracts: In this study, to investigate the synergistic effect and appropriate input threshold of urea ammonium nitrate solution (UAN) on urea under drip irrigation of maize, a two-year field located experiment was conducted in Qian’an County in semi-arid region of Jilin Province in 2020 and 2021, using UAN as the tested material. The nitrogen (N) fertilizer application treatments were included 0, 100, 140, 180, 220, and 260 kg(N)∙hm⁻², and optimized urea application (Urea) was used as control. And their differences on maize yield, its components, dry matter accumulation, leaf area index, N use efficiency, soil N content and soil N apparent balance were studied. The results showed that maize yield was significantly affected by N fertilizer application rates, which increased with the increase of N fertilizer application and reached the yield plateau under 180 kg∙hm⁻² of N fertilizer application rate. Maize yield was 2.02% higher than that of Urea treatment in average two years under this N fertilizer application rate. The yield advantage of UAN treatments were mainly attributed to increasing kernels number per ear and 100-kernel weight. Leaf area index, dry matter accumulation amount, translocation amount and contribution rate of dry matter accumulation to grain after silking stage were all increased with the increase of N fertilizer application rate during the various growth stages of maize. The increasing values were no longer significant when the N fertilizer application rate increased to 180 kg∙hm⁻², and were all higher than those of Urea treatment. N recovery efficiency, agronomic efficiency and partial factor productivity decreased with the increase of N fertilizer application rate. And these values of UAN treatment were increased by 4.45 percentage points, 2.09 kg∙kg⁻¹ and 2.09 kg∙kg⁻¹ than those of Urea treatment under the same N fertilizer application rates, respectively. Compared with urea treatment, UAN treatment under the same N fertilizer application rates increased the inorganic nitrogen content in the 0~40 cm soil layer and decreased the inorganic nitrogen content in the 40~100 cm soil layer under the same N fertilizer application rates, resulting in a 7.51% reduction in N apparent loss compared to urea treatment. N fertilizer application rate of 188.90 kg∙hm⁻² is appropriate by simulating between N fertilizer application rate, maize yield, N recovery efficiency and soil N apparent loss, respectively. And the corresponding maize yield, N recovery efficiency and soil N apparent loss would be 11 863.42 kg∙hm⁻², 44.21% and 151.79 kg∙hm⁻², respectively. Compared with Urea treatment, maize yield and N recovery efficiency increased by 4.78% and 7.61 percentage points respectively, and N fertilizer application rate and N apparent loss decreased by 14.14% and 29.55% respectively. The N fertilizer application rate was calculated as 179~198 kg∙hm⁻² using a 95% the highest theoretical yield as the confidence interval. In conclusion, compared with the application of urea, the application of UAN can reduce the total N input and N apparent loss, and achieve higher maize yield and N use efficiency. It is an effective way to achieve nitrogen reduction and efficiency enhancement of maize in semi-arid region of Northeast China.