In order to reveal the effects of phosphogypsum addition on soil fertility and enzyme activity in the cultivated layer of saline-sodic paddy fields in the west of Songnen Plain, this study reclaimed the former saline-alkaline wasteland as an experimental paddy field and set up four phosphogypsum addition treatments. They are 0 t/hm₂ ^{(CK), 15 t/hm}₂ ^{(G15), 30 t/hm}₂ ^{(G30) and 45 t/hm}₂ ^{(G45), respectively.} The results show that: (1) The addition of phosphogypsum at 15-45 t/hm^₂ significantly improved soil fertility in saline-sodic paddy field. Soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), C/N and C/P 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, with the largest increase in each indicator coming from the G45 treatment. (2) The activities of different types of enzymes were significantly different among different treatments. The activities of urease, leucine aminopeptidase and alkaline phosphatase were highest in G45 treatment, the activities of sucrase were highest in G30 treatment, and the activities of cellulase were highest in CK treatment. 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 appeared in G15 treatment. 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 activity (F≥4.6, p<0.05). Phosphogypsum indirectly affects the activities of cellulase, leucine aminopeptidase and alkaline phosphatase by changing the contents of TN and SOC, and indirectly affects the activities of sucrase and urease by changing the contents of TN. The research results can provide support for further improving the mechanism of phosphogypsum improving saline-sodic paddy field soil.