Yellow River Delta, which is surrounded by Bohai Sea to the north and Laizhou Bay to the east, is one of the three largest deltas in China. However, large amounts of water and salts that were brought by penetration of water in the Yellow River and encroachment of sea water resulted in the rise of groundwater level and salinization. Under the influence of strong evaporation, the soil degraded into saline soil. Currently, up to around 1670 km² of land have turned into saline soil in the Yellow River Delta. Thus, it is urgent to ameliorate the saline soil for the assurance of ecological security and coordinated development of economy and ecology. Treated paper mill effluent was low in salinity and high in organic matter, and could be used to restore saline soil. Irrigation with treated paper mill effluent could enrich soil nutrients, improve soil activity and leach soil salts, and meanwhile it could solve the disposal problem of paper mill effluent. Thus, different depths (viz. 5, 10, 15 cm and 20 cm) of treated paper mill effluent were irrigated for remediation of saline soil in this study. In addition, anti-waterlogging ditches were built around the plots for discharging leached soil salts. Soil enzyme activities, which can reflect soil fertility, are important parameters for evaluating soil quality. Five enzymes, viz. urease, phosphatase, sucrase, dehydrogenase and catalase, were used to evaluate the effectiveness of saline soil remediation. Under the multi-influences of soil water content, temperature, aeration, nutrients and salts, soil enzyme activities were higher in soil layers of 0-10 cm and 10-20 cm than those in soil layer of 20-30 cm. Irrigation did not change the distribution of soil enzymes in the different soil layers. The improvements of urease and phosphatase mainly achieved in soil layer of 0-10 cm, and those of sucrase, dehydrogenase and catalase mainly achieved in both soil layers of 0-10 cm and 10-20 cm. Irrigation rate of 20 cm was the most effective in improving soil enzyme activities in lower soil layer (20-30 cm). The five enzymes also presented regular seasonal dynamics, all reaching the highest level in August. Irrigation with treated paper mill effluent, salinity (2080 mg/L) of which was very low compared to the present saline soil (2.31%), was effective in reducing soil salinity. Thus, inhibition of soil salinity to enzymes was reduced. In addition, irrigation with treated paper mill effluent introduced abundant of substrates for enzymes. Consequently, all the five enzyme activities were significantly improved. In general, higher irrigation rates were more effective in the improvement of enzyme activities and 20 cm turned out to be the optimum irrigation rate. Compared to the control, the activities of urease, phosphatase, sucrose, dehydrogenase and catalase were improved by 70.0%, 30.9%, 56.2%, 135.2% and 20.84%, respectively. The results also showed that in the studied saline soil, the five soil enzyme activities presented significantly negative correlation with soil salinity, sodicity and microbial metabolic quotient (qCO₂), and significantly positive correlation with available phosphorus and microbial biomass carbon, while no significant correlation with soil organic matter and available nitrogen due to the different dynamics over time after irrigation.