In order to improve the mechanized spraying effect of stunted and close planting cotton, an airassisted cotton boom sprayer with separating row and spraying in inside and upper canopy by combining separating row and spraying inside canopy technology with airassisted spraying technology was designed. This sprayer braced the droplet diffusion space of inside cotton canopy by row separator and improved droplet diffusion range and distribution uniformity by airflow disturbance and transport of inside and upper cotton canopy. The structure of airassisted boom sprayer and the design of row separator were described. The airassisted system was designed by combining substitution principle with end speed principle. The design method of air duct outlet was elaborated. The airassisted cotton boom sprayer was tested by wind field test and spray deposit distribution test. Wind field test showed wind velocity of the air flow field was big enough and welldistributed. Spray deposit distribution test showed the front leaves average droplet coverage of inside cotton canopy was 65.30% and the back leaves average droplet coverage of inside cotton canopy was 3983%. The biggest difference of front leaves average droplet coverage among upper canopy, middle canopy and lower canopy was 10.25% and the biggest difference of back leaves average droplet coverage among upper canopy, middle canopy and lower canopy was 11.75%.The spray deposit distribution uniformity of the whole cotton canopy was good and the spraying effect was improved obviously.