Abstract: This study aims to explore the effects of the reduced phosphorus fertilizer with magnesium-modified biochar on the growth, phosphorus utilization, and yield of peanut under deficit irrigation in the windy sandy areas. A field experiment was conducted at the Jianping Irrigation Experimental Station in Chaoyang City, Liaoning Province, China, from May to October in 2023 and 2024. The peanut variety “Baisha 1016” was used as the material in the field experiment. And the corn straw biochar was purchased from Shenyang Longtai Biological Engineering Co., Ltd, China. The experiment was carried out as a split-plot design. A systematic investigation was made to evaluate the effects of the irrigation and biochar treatments on the SPAD value, net photosynthetic rate, leaf area index, dry matter weight, soil available phosphorus content, plant phosphorus utilization, and yield of peanut. The main plot was taken from the rain-fed (RF, and no irrigation) and deficit irrigation (MD. During the flowering and pod setting stages, the plot was re-watered to 90% field capacity when soil water content dropped below 55% field capacity, and no irrigation was applied during other growth stages. The subplot was taken as the reduced phosphorus with biochar applications (P₁B₀, 105 kg/hm² phosphorus fertilizer (conventional phosphorus rate) with no biochar application; P₁B₁, 105 kg/hm² phosphorus fertilizer applied with 10 t/hm² magnesium-modified biochar; P₂B₁, 78.8 kg/hm² phosphorus fertilizer (25% phosphorus reduction) applied with 10 t/hm² magnesium-modified biochar; P₃B₁, 70.4 kg/hm² phosphorus fertilizer (33% phosphorus reduction) applied with 10 t/hm² magnesium-modified biochar). The results showed that both deficit irrigation and reduced phosphorus with biochar treatments significantly improved the peanut growth and phosphorus utilization over different growth stages. Compared with the RF treatment, the MD treatment increased the SPAD value and leaf area index of peanuts in the seedling, flowering, pod-setting, and pod-filling stages by 4.5%, 7.0%, 8.4% and 8.1% (SPAD value) and 19.4%, 19.8%, 23.3% and 31.2% (leaf area index), respectively (two-year average). The MD treatment increased the pod yield and kernel yield of peanut by 10.5% and 11.5% (two -year average), respectively, compared with the RF treatment. The P₂B₁ treatment increased the dry weight by 25.7% to 46.3% (two-year average) over the growth stages, compared with the P₁B₀ treatment. The P₂B₁ treatment increased the soil available phosphorus content during the seedling, flowering, pod-setting, and pod-filling stages by 14.7%, 14.1%, 34.9%, and 39.2%, respectively, in 2023, compared with the P₁B₀ treatment. The P₂B₁ treatment increased the plant phosphorus accumulation during the seedling, flowering, pod-setting, and pod-filling stages by 46.7%, 58.9%, 55.6%, and 46.6%, respectively (two-year average). Furthermore, there was an increase in the pod yield, kernel yield, and 100 fruit mass by 11.2%, 12.4%, and 18.4%, respectively, compared with the P₁B₀ treatment (two-year average). Among them, the MDP₂B₁ treatment achieved the highest peanut yield, indicating a 23.5% higher peanut yield than the conventional treatment (RFP₁B₀) (two-year average). The principal component analysis indicated that the combination of deficit irrigation and 25% phosphorus reduction with 10 t/hm² magnesium-modified biochar (MDP₂B₁) enhanced the soil available phosphorus content, and then promoted the phosphorus utilization and the peanut growth of the peanuts plant, ultimately increasing the high peanut yield. These findings can provide the theoretical foundation for the efficient utilization of phosphorus fertilizer in the peanut field of the sandy areas.