Changing the distribution of soil roots to absorb deep soil water may be an important strategy for plants to avoid drought. Lathyrus sativus is a kind of legume crop with strong stress resistance. We explored the effects of different irrigation modes (Full irrigation, FI; Partial root drying, PRD; No irrigation, NI) on soil moisture, root distribution, gas exchange, water use efficiency and yield of Lathyrus sativus at different periods after sowing. The results showed that: (1) 89.8%, 86.9% and 84.9% of the roots of L. sativus grew in 0-20 cm topsoil under FI, PRD and NI treatments, respectively. Drought stress increased the proportion of roots in deep soil to 13.05% and 15.07% under PRD and Ni treatments, which provided guarantee for roots to absorb deep soil water and avoid the influence of drought on L. sativus. (2) During the whole growth period, soil drought significantly reduced the net photosynthesis, transpiration and stomatal conductance; At 60 days after planting, PRD and NI treatment significantly improved the instantaneous water use efficiency of leaves, which was 21.4% higher than that of the control (FI treatment) and 14.9% higher than that of NI treatment. (3) The drought reduced plant height, first pod height, average pod number and seed number of L. sativus. Drought also led to a significant decrease in aboveground biomass and root system, but a significant increase in root shoot ratio; PRD treatment had no significant effect on pod length, pod weight and seed weight per pod. Drought reduced the average grain yield loss of PRD and NI reached 53% and 63%. The study found that under drought stress conditions, L. sativus could alleviate drought by increasing the proportion of roots in the deep soil, absorbing more water in the deep soil, significantly increasing the ratio of root to shoot, and significantly improving the instantaneous water use efficiency of leaves in the reproductive growth period. These results provide a reference for the cultivation of L. sativus in arid areas.