Because of the ubiquitous linear correlation between attribute indexes, applying the Euclidean distance formula to calculate the distance from each evaluation object to positive and negative ideal solution doesn’t work. The distance calculation is improved by taking the advantages of Mahalanobis distance’s unaffected by dimension, and it’s immuned from indexes correlation. Topsis improved has the scientificalness and rationality. Finally, the concrete application of improved Topsis in the supplier selection is given.