Aiming at the salinity stress problems in rice root growth in the paddy soil of coastal saline-alkali soil, and the consequent low nutrients use efficiency, afield experiment was conducted to study the effects of combined application of organic and phosphate fertilizers on root growth in different growth stages, yield and nutrients use efficiency of rice in coastal saline soil. The experiment had two factors, including three carbon levels:(1) C0, no carbon; (2) C1:low carbon, 450 kg/hm²; (3) C2:high carbon, 900 kg/hm², and three phosphate fertilization (P) levels:(1) P0:no P; (2) P1:low P, P₂O₅64 kg/hm²; (3) P2:high P, P₂O₅128 kg/hm².Results showed that the total root surface area of T5 treated with low carbon and low P was significantly higher than those of high carbon and low phosphorus treatments (T7) and high carbon and high phosphorus treatments (T8), which increased by 25.2% and 30.2%, respectively. The total root volumes of low carbon treatments (T5, T6) were significantly higher than that of high carbon treatment in the harvest stage of rice. The rice yield and biomass of T5 were 10 245 and 9 550 kg/hm², respectively, and higher than others treatments. High seed setting rate was the reason for the highest yield of rice treated with low carbon and low P (T5) treatment. P accumulation of rice in low carbon and low P (T5) treatment was significantly higher than those in T6, T7 and T8 treatments, which were higher than 13.9%, 27.8% and 31.2%, respectively. The fertilizer contribution rate (FCR) and agronomic efficiency (AE) of P fertilizer under T5 treatment were significantly higher than others treatments. The partial fertilizer productivity (PFP) P fertilizer was significantly higher at low P input than that of high P input. In conclusion, compared with only inorganic fertilizer, combined application of organic fertilizer with P fertilizer could significantly promote rice root growth, increase rice yield and agronomic efficiency of P fertilizer in coastal saline soil. Low carbon and low P (T5, C450 kg/hm² and P₂O₅ 64 kg/hm²) treatment was the most favorable for rice root growth in coastal saline soil.