In order to determine the optimal spacing of underground drainage pipes and promote the improvement of saline-alkali soil in Hetao irrigation area, a field experiment with rotation irrigation was conducted, soil salt contents, alkalization degrees and fertility indexes under 10, 20 and 30 m spacings were determined and compared. The results showed that soil salt contents of 0-10 cm soils with 10, 20 and 30 m spacings were 17.22, 17.74 and 17.17 g/kg respectively before leaching, and were 10.78, 11.56 and 11.95 g/kg respectively before autumn irrigation. Soil desalination rates of different spacings were as follows: 10 m>20 m>30 m. Soil alkalization indexes increased first and then decreased after irrigation. pH of 0-10 cm soils with 10, 20 and 30 m spacings before autumn irrigation decreased by 4.47%, 3.72% and 2.96% respectively compared with that after the third leaching, indicating that small spacing was more effective to alleviate soil alkalization. But underground pipe drainage could lead to severe soil nutrient loss, which was in an order of 10 m >20 m>30 m.