Rural domestic reclaimed water (RDRW) brings in a large amount of nitrogen that could affect the nitrogen supply capacity of soil and the absorption and utilization of crops. Four kinds of irrigation water sources (primary and secondary treated water R1 and R2, purified water R3, and river water CK) and three kinds of fertilization gradients (10%, 30%, and 100% conventional nitrogen fertilizer reduction of N1, N2, and N0) were set up to study the effects on the efficient utilization and availability of nitrogen in paddy rice. ¹⁵N tracer technology combined with fertilizer equivalent methods was used. The results showed that the nitrogen absorbed and utilized for soil and crop systems mainly came from fertilizer nitrogen (NF), soil nitrogen (NS), and reclaimed water nitrogen (NRW). NS was the main source of nitrogen uptake by plants. NRW use efficiency (RWNUE) was not directly proportional to the nitrogen concentration in RDRW, while NRW residue rate (RWNRE) was inversely proportional to it. Compared with CK, the absorption and utilization of nitrogen were inhibited, and the contribution rates of NF and NRW were both decreased under RDRW irrigation. Under N1 and N2, the NF relative substitution equivalent (RFE) of R1, R2, and R3 was 28.1% and 56.3%, 13.6% and 46.6%, 1.3% and 5.4%, respectively. Since reducing the fertilization gradient can effectively improve NRW availability, 30% and 10% nitrogen reduction fertilization were recommended for R1 and R2 irrigation, which can fully utilize the effectiveness of nitrogen in reclaimed water in paddy fields.