Abstract: The study aims to find out the effect of different degrees of regulated deficit irrigation on the yield and quality of island cotton. The authors used field plot test and took the normal irrigation water as control, set the mild regulated deficit irrigation (RDI) (75% of normal irrigation water), moderate (50% of normal irrigation water) RDI and severe RDI (25% of normal irrigation water), to study the adjustment effect of RDI on the yield and quality formation of island cotton. The results showed that mild RDI had no significant effect on cotton production quality, and moderate and severe RDI reduced lint yields by 10.8% and 25.4% , respectively, mainly for the significant decrease of single boll weight per plant. Mild and moderate RDI had some effects on lint, but severely RDI made lint dropped significantly. Mild and severe RDI slightly cut down the fiber length, while moderately RDI increased fiber length. Fiber specific strength and incompatible micronaire value increased with the rise of RDI degree, but the difference was not significant. From the perspective of different boll formation parts, mild RDI increased central cotton boll weight, fiber length and specific strength, and significantly reduced the upper boll weight and fiber length. Moderate and severe RDI significantly reduced boll weight and lint in the central part, moderate RDI increased fiber specific strength of central cotton bolls, and severe RDI significantly increased fiber specific strength of lower cotton bolls. Mild, moderate and severe RDI significantly increased the incompatible micronaire value of the central, lower and upper boll fiber respectively.