Abstract: A field study was conducted to determine the relationships between Crop Water Stress Index(CWSI), yield, and quality of cotton under drip irrigation with mulch. The lower baseline formula of cotton CWSI under full water supply at different growth stages of cotton was determined. The relationship between CWSI and vapour saturation deficiency (VPD) was analyzed. And the applicability and and optimal time for assessing crop water status using CWSI were put forward. Based on regular observation of cotton CWSI at cotton growth stage under drip irrigation with mulch in different water treatments, the realationships between arerage values of CWSI and water consumption, lint yield, water use efficiency, bell weight, gin turnout and velvet length were studied. The results showed that a suitable CWSI value of 0.4 was proposed for irrigation scheduling of dripped cotton with an infrared thermometer method except gin turnout. Hence, the CWSI could be used in cotton field water management.