To solve the problems of the AMR-WB+ (Extended Adaptive Multi-Rate-WideBand) semi-open-loop coding mode selection algorithm, features for ACELP (Algebraic Code Excited Linear Prediction) and TCX (Transform Coded eXcitation) classification are investigated. 11 classifying features in the AMR-WB+ codec are selected and 2 novel classifying features, i.e., EFM (Energy Flatness Measurement) and stdEFM (standard deviation of EFM), are proposed. Consequently, a novel semi-open-loop mode selection algorithm based on EFM and selected AMR-WB+ features is proposed. The results of classifying test and listening test show that the performance of the novel algorithm is much better than that of the AMR-WB+ semi-open-loop coding mode selection algorithm.