Reclaiming the former saline-sodic wasteland as an experimental paddy field, four phosphogypsum addition treatments were set up, namely 0 (CK), 15 (G₁₅), 30 (G₃₀), and 45 t/hm² (G₄₅), to study the effects of phosphogypsum addition on soil fertility and enzyme activity of the cultivated layer in saline-sodic paddy fields in the western Songnen Plain. The results showed that: 1) The soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), carbon to nitrogen ratio (C/N) and carbon to phosphorus ratio (C/P) of phosphogypsum treatments (G₁₅, G₃₀ and G₄₅) were increased by 14.59%-42.86%, 14.11%-33.25%, 11.62%-26.41%, 15.53%-115.47%, 0.66%-7.55% and 4.43%-10.78%, respectively, compared with CK treatment, with the largest increase all under G₄₅. 2) The activities of soil enzymes were significantly different among different treatments, the activities of urease, leucine aminopeptidase and alkaline phosphatase were highest under G₄₅, the activity of sucrase was highest under G₃₀, and the activity of cellulase was highest under CK. With the increase of phosphogypsum addition, the activities of sucrase and urease continued to increase, while the activities of cellulase, alkaline phosphatase and leucine aminopeptidase first decreased and then increased, and the minimum values all appeared under G₁₅. The geometric mean of enzyme activities was G₄₅>CK>G₃₀>G₁₅. 3) Under phosphogypsum addition condition, the contents of TN, SOC, AP and TP had the greatest effects on enzyme activities (Pseudo-F≥4.6, P<0.05), phosphogypsum indirectly affected the activities of cellulase, leucine aminopeptidase and alkaline phosphatase by changing TN and SOC contents, and the activities of sucrase and urease by changing TN content. Overall, phosphogypsum addition significantly enhances the soil fertility of the cultivated layer in saline-sodic paddy fields, thereby indirectly reshaping the activity characteristics of different soil enzymes and the geometric mean of enzyme activities by altering TN and SOC contents.