The effects of irrigation, fertilization, planting density and filmmulching were comprehensively analyzed in order to develop a universal Logistic model of cotton relative leaf area index based on the relative growing degree days as the independent variable. The relationship between the maximum leaf area index of cotton and irrigation amount, nitrogen application rate, planting density and yield during the whole growth period was studied. The results showed that the growing degree days of the maximum leaf area index of cotton with film-mulching was about 1400℃, and growing degree days of the maximum leaf area index of cotton without film-mulching was about 1600℃. The maximum leaf area index of cotton was increased first and then decreased with water consumption and nitrogen application, and there was a significant positive correlation between planting density and maximum leaf area index. The maximum leaf area index can be described accurately through considering the effects of irrigation amount, nitrogen application rate and planting density. When the maximum leaf area index was 4.93, the maximum seed cotton yield was 6066.2kg/hm2. The results of relative analysis showed that the change trend of relative leaf area index of cotton with and without film-mulching was nearly the same, and the corresponding model parameters were also the same. A unified Logistic model of relative leaf area index of cotton with and without film-mulching was established. The research result can provide a method for scientific planting and fine management of cotton, and also can provide a reference for modeling other crops in different management and different regions.