The invention discloses a rapid lossless measuring method for a cotton leaf area index. The rapid lossless measuring method includes calculating the leaf area index according to combination of cotton canopy transmission and blade scattering, and correcting the leaf area index by a correction model to acquire the corrected leaf area index. The correction model is LAI=0.84*LAIAccuPAR+0.0879, the LAI refers to the corrected leaf area index, and the LAIAccuPAR refers to the leaf area index acquired by calculation before correction. A conventional destructive direct measuring method is replaced by the indirect measuring method, the leaf area index acquired by calculation according to the combination of cotton canopy transmission and blade scattering is corrected by the correction model, and the real leaf area index of cotton groups in the field is figured out through back calculation. The method can be used for rapidly and effectively estimating a real value of the leaf area index of the cotton groups in the field without losses, estimating and judging the growth trend of the cotton groups and applying corresponding planting measures for field management.