Abstract: Direct recognition of the redundant methods of optimal combining forecasts with non-negative weights from information matrix or covariance matrix of forecasting errors can simplify the computation of optimal weights and therefore help the practical application of optimal combination forecasting.In this paper,a supplement to the criteria for determining the redundant methods is proposed and illustrated to be more applicable than the previous criteria.Furthermore,the limitations in the present study of combining forecasts with non negative weights and redundant methods are discussed and further research of robust combining forecasts with non-negative weights,which is adaptable to structural changes is proposed.