<p>Antibiotic accumulation in saline-alkaline soils is an escalating problem in agriculture and is receiving increasing attention. Biochar amendment for saline-alkaline land is an effective approach to improve soil quality. However, the effects of modified biochar on the antibiotic adsorption mechanism and occurrence forms in saline-alkaline land remain unclear. In this study, phosphate acid-modified biochar (PBC) was used as the research material, and its effects on the adsorption mechanism, occurrence forms, and bioavailability of oxytetracycline (OTC) in salinized-alkaline soil were investigated. The results showed that the adsorption of OTC by saline-alkali soil amended with 5% PBC reached equilibrium within 24&#xa0;h, with an adsorption capacity ranging from 2.05 to 4.14&#xa0;mg/g. The pseudo-second-order kinetic model (R² = 0.9754–0.9422) was more suitable for describing this adsorption process, indicating that chemical adsorption dominated. Considering the complex compositions of saline-alkali soil and PBC provided multiple adsorption sites with uneven energy distribution for OTC adsorption (a characteristic of multi-layer adsorption), the Freundlich model (R² = 0.9956–0.9634) better fitted the adsorption behavior. PBC effectively reduced the bioavailability of OTC by improving soil properties, including decreasing the pH value of saline-alkali soil, reducing soluble salt content, and increasing organic matter content (<i>p</i> &lt; 0.01). The bioavailability of OTC decreased by 16.80–25.23% (<i>p</i> &lt; 0.05) in Na₂SO₄ saline-alkali soil and by 11.56–15.49% (<i>p</i> &lt; 0.05) in Na₂CO₃ saline-alkali soil. The application of PBC in the remediation of OTC-contaminated saline-alkali soil not only enhanced the adsorption capacity for OTC but also improved the physicochemical properties of saline-alkali soil, thereby achieving the synergistic effect of pollution control and soil improvement, as well as effective control of environmental risks.</p>