A rational plant population is an important attribute to high yield of cotton, because it can provide a beneficial micro environment within the canopy for plant growth and development as well as yield formation. A 2-yr field experiment was conducted to determine the optimal plant density based on cotton yield in relation to the canopy micro environment(canopy temperature, relative humidity and light transmittance). Six plant densities(1.2-5.7 plants m-2) were arranged with a completely randomized block design. The highest cotton yield(1 507 kg ha-1) was obtained at 3.0 plants m-2 due to more bolls per unit ground area(79 bolls m-2), while the lowest yield(1 091 kg ha-1) was obtained at 1.2 plants m-2. Under the moderate plant density(3.0 plants m-2), there was a lower mean daily temperature(MDT, 27.1℃) attributing to medium daily minimum temperature(Tmin, 21.9℃) and the lowest daily maximum temperature(Tmax, 35.8℃), a moderate mean canopy light transmittance of 0.51, and lower mean daily relative humidity(MRH) of 79.7% from June to October. The results suggest that 3.0 plants m-2 would be the optimal plant density because it provides a better canopy micro environment. A rational plant population is an important attribute to high yield of cotton, because it can provide a beneficial micro environment within the canopy for plant growth and development as well as yield as well. A 2-yr field experiment was conducted to determine the optimal plant density based on cotton yield in relation to the canopy micro environment (canopy temperature, relative humidity and light transmittance). Six plant densities (1.2-5.7 plants m-2) were arranged with a completely randomized block design. The highest cotton yield (1 507 kg ha-1) was obtained at 3.0 plants m-2 due to more bolls per unit ground area (79 bolls m-2), while the lowest yield (1 091 kg ha-1) was obtained at 1.2 plants m-2. Under the moderate plant density (3.0 plants m-2), there was a lower mean daily temperature (MDT, 27.1 ° C) attributing to medium daily minimum temperature (Tmin, 21.9 ° C) and the lowest daily maximum temperature , a moderate mean canopy light transmittance of 0.51, and lower me an daily relative humidity (MRH) of 79.7% from June to October. The results suggest that 3.0 plants m-2 would be the optimal plant density because it provides a better canopy micro environment.