Biochar modifications and fortifications are used to increase soil nutrient retention and increase crop yields, but the effects on phosphate (Pi) availability for plants are unknown for saline-alkaline soils. We used peanut shells (Peanut-S) as the starting material for biochar (Biochar) and MgO-modified biochar (MgO-Biochar) to study the Pi adsorption mechanisms under field conditions. Compared with Biochar and Peanut-S, MgO-Biochar adsorbed Pi in aqueous solution 20% better and reached equilibrium at twice the rate at twice the rate. MgO-Biochar also showed better Pi adsorption in saline soils and the maximum Pi adsorption capacity was 1.46 times higher than Biochar. We found that electrostatic attraction, precipitation and exchangeable anions contributed to the adsorption of phosphate and MgO-Biochar possessed the greatest adsorption capacity. MgO-Biochar application increased the soil available P content and resulted in higher rice yields in field experiments in the saline soil conditions found in the Yellow river delta region. Overall, our study offers a theoretical and practical foundation for the remediation of saline soil using MgO-modified biochar.