<p>Recently, Diammonium phosphate (DAP, (NH₄)₂HPO₄) has gradually replaced fused calcium-magnesium phosphate (FCMP) as the primary source of phosphorus in sugarcane fields in China. FCMP is a thermally processed phosphate fertilizer primarily containing α-Ca₂(PO₄)₂ and CaMgP₂O₇. Despite its possible impacts on sustainable sugarcane production, the effect of this substitution on accelerating soil acidification has not yet been investigated. In this study, a three-year-long experiment was performed in Tuolu Town of Jiangzhou District, and field experiments were conducted with five treatment groups with different substitution ratios of DAP replacing FCMP (0%, 25%, 50%, 75%, and 100% of P₂O₅). The results indicated that the replacement of FCMP with DAP decreased soil pH. The decrease in soil pH is closely related to the increase in soil exchangeable aluminum and the decrease in soil exchangeable calcium and magnesium. In acidic soil, FCMP can release and increase Ca²⁺ and Mg²⁺ to compensate for the losses of Ca²⁺ and Mg²⁺ over time. With an increase in the proportion of DAP replacing FCMP, soil exchangeable Ca²⁺ and Mg²⁺ decreased and soil exchangeable Al³⁺ increased simultaneously. Complete replacement of FCMP with DAP increased soil exchangeable aluminum by 0.23 cmol kg⁻¹ yr⁻¹. The accumulation of soil exchangeable Al³⁺ and loss of exchangeable (Ca²⁺ + Mg²⁺) had an approximately 1:1 linear negative correlation. Our study shows that applying FCMP further prevents soil acidification in acidic sugarcane field, but replacing FCMP with DAP leads to a decrease in soil pH and an increase in soil exchangeable aluminum. Therefore, FCMP should be favored in acidic sugarcane fields to mitigate acidification. These results can provide an important reference for decision-making in China’s fertilizer and sugarcane industries.</p>