Introducing defects into porous metal-organic frameworks is very important for improving adsorption performance. Due to their simplicity of synthesis and improved efficiency in MOFs applications, Quasi-MOFs have recently been considered as underutilized variant of large-scale, fundamentally deficient MOFs with substantial amounts of unsaturated metal sites to offset the drawbacks of MOFs. Herein, a quasi-MOF was produced using a water-stable MOF (MOF-303) which is based on one of the best phosphate-interacting metals (Al³⁺), that underwent controlled thermolysis in an air atmosphere at various temperatures between 430 and 470 °C. The acquired Quasi-MOF-303 underwent thorough characterization. The highest adsorption value of 488 mg·g⁻¹ for phosphate was achieved at 298 K at an intact pH value of medium(which is roughly 25 times more than the intact structure). The experimental results demonstrated an exothermic spontaneous mechanism for the phosphate adsorption process that fit the pseudo-second-order and Langmuir isotherm models. The findings show that defect sites are crucial for improving phosphate uptake capability. This is further supported by several advanced characterizations, which resulted in the highest reported phosphate adsorption rate among all known adsorbent MOFs.