Abstract: In order to develop the optimal coupling model of water and nitrogen of watermelon under limited irrigation in gravel-mulched field, a field experiment with split-plot design was conducted to study the effects of supplementary irrigation volume, nitrogen fertilization, and their interactions on the growth, yield, quality and water and nitrogen use efficiency of watermelon with 4 supplementary irrigation levels (W: 0, 35, 70, and 105 m³·hm^-2 ) in main plots and 3 nitrogen fertilization levels (N: 0, 120, and 200 kg N·hm^-2) in subplots. The results showed that the photosynthetic rate, yield, and water and nitrogen use efficiency of watermelon increased with the increasing supplementary irrigation, but the nitrogen partial productivity and nitrogen use efficiency decreased with increasing nitrogen fertilization level. The photosynthetic rate and quality indicators increased with increasing nitrogen fertilization level as the nitrogen rate changed from 0 to 120 kg N·hm^-2, but no further significant increase as the nitrogen rate exceeded 120 kg·hm^-2. The interactive effects between water and nitrogen was significant for yield and water and nitrogen use efficiency of watermelon, supplementary irrigation volume was a key factor for the increase yield compared with the nitrogen fertilizer, and the yield reached the highest for the W₇₀N₂₀₀ and W₁₀₅ N₁₂₀ treatments, for which the yield increased by 42.4% and 40.4% compared to CK. Water use efficiency (WUE) was improved by supplementary irrigation and nitrogen rate, the WUE of all nitrogen fertilizer treatments were more than 26 kg·m^-3 under supplemental irrigation levels 70 m³·hm^-2 and 105 m³·hm^-2. The nitrogen partial productivity and nitrogen use efficiency reached the highest in the treatment of W₁₀₅N₁₂₀. It was considered that under the experimental condition, 105 m³·hm^-2 of supplementary irrigation plus 120 kg·hm^-2 of nitrogen fertilization was the optimal combination of obtaining the high yield and high efficiency.