Abstract: Abstract: With the long-term use of under mulched drip irrigation, the problem of salt accumulation in farmland soil has attracted increasing attention. In order to explore the changes of soil salt content in a cotton field under mulched drip irrigation with subsurface pipes drainage, a subsurface pipes drainage experiment was conducted in the saline-alkali soil of Xinjiang 122 group in 2013 and 2014. The buried depth of the subsurface pipes is 2.2 m, and the spacing is 48 m. The changes of soil salt content in the 0-200 cm soil layer were monitored through under mulched drip irrigation. Sampling points were set at 8, 16, and 24 m perpendicular to the subsurface pipes and the sampling time was set to the cotton seedling, flowering, and boll opening periods of 2013 and 2014, respectively. Soil electrical conductivity (EC) was detected by an electrolytic conductivity meter at a soil-to-water ratio of 1:5. Based on the HYDRUS-2D model, a mathematical model of soil salt content changes in cotton fields under mulched drip irrigation with subsurface pipes drainage was established, and the HYDRUS-2D simulation results and experimental data were compared. The range of the statistical analysis index salt content RMSE and NSE are 0.39-1.26 g/kg and 0.66-0.97, respectively. They were within the acceptable range, which verified the reliability of the numerical simulation method. On this basis, the model simulated and analyzed the changes of salt content with time in 0-80 cm soil layer, the desalination rate of root layer, and the desalination rate of 0-200 cm soil layer in the salt content of 2013 and 2014 during cotton growth and the autumn salt return stage. The simulation results showed that under mulched drip irrigation during the growth period of cotton, the soil salt content continued to decrease, and the soil surface began to return after the harvest. The soil salt content in the cotton boll opening period in 2013 and 2014 was compared with the initial salt content, the average desalination rate of the 0-80cm soil layer reached 41.11% and 55.56%, respectively. And the total salt content of the 0-80cm soil layer under and between the mulching films decreased by 14.05% and 17.88%, respectively. After the cotton was harvested, salt returned to the upper layer of soil. However, it still showed a decrease compared with the initial salt content. The average salt content of 0-80 cm soil decreased by 5.55% and 10.15%, and the average salt content of 0-200 cm decreased by 2.58% and 4.96%. These results showed that the total salt content in soil showed a decreasing trend by using under mulched drip irrigation with subsurface pipes drainage mode. The average soil desalination rate of 0-8 m (2.97% and 5.71%) from the subsurface pipes was significantly higher than that of 8-16 m (2.44% and 4.64%) and 16-24 m (2.32% and 4.52%), but the difference between the latter two was small. The area close to the subsurface pipes had a low soil salt content, while the area far away from the subsurface pipes had a higher soil salt content. According to the law of solute transport and the principle of salt balance, the salt in the soil would change from the area with more salt to less salt over time. As a result, the overall soil salt content decreased, and the root layer salinity decreased more obviously. The results revealed the changing law of soil salt content in the cotton fields under mulched drip irrigation with subsurface pipes drainage, aiming to provide theoretical support and scientific guidance for the popularization and application of subsurface pipes drainage technology and under mulched drip irrigation in the inland arid area of Northwest China.