In order to ascertain the alkaline phosphatase (ALP) activity and its relationship with soil properties in saline-sodic soils during reclamation, a study was conducted in a saline-sodic soil reclaimed by cropping wolfberry (Lycium barbarum L.) with drip irrigation, in Ningxia Plain, Northwest China. The soil ECe, pH and SAR in 0-30 cm were 12.3 dS m(-1), 9.4 and 44.1 (mmol/L)(0.5), respectively. Soil transects with different planting years were intensively sampled, which had a wide gradient of salinity and sodicity. Ranging from 1.1 to 42.4 mu g g(-1) h(-1), soil ALP activities increased with the increasing planting years, and showed a large spatial variability within transect. The higher ALP activities were always found beneath the drip emitters. More soil physicochemical properties became related significantly to the ALP activities as the planting years increased, indicating that the ALP activities could be better predicted by other properties after reclamation. Path analyses showed that the negative direct effects of soil pH on ALP activities became clearly dominant as the planting years increased. The positive effects of organic matter on ALP activities exerted indirectly, mainly through pH, total N, and available P. Soil ALP activities decreased exponentially with pH, which varied from 7.38 to 10.00. Our findings demonstrated that soil pH was the limiting factor for improving soil ALP activities in this saline-sodic soil, and after three planting years, soil biological activities and fertility level increased significantly.