1. A method, comprising: at a device having one or more processors and memory: establishing a first-level Deep Neural Network (DNN) model based on unlabeled speech data, the unlabeled speech data containing no speaker labels and the first-level DNN model specifying a plurality of basic voiceprint features for the unlabeled speech data; establishing a second-level DNN model by tuning the first-level DNN model based on labeled speech data, the labeled speech data containing speech samples with respective speaker labels, wherein the second-level DNN model specifies a plurality of high-level voiceprint features; using the second-level DNN model, registering a first high-level voiceprint feature sequence for a user based on a registration speech sample received from the user; and performing speaker verification for the user based on the first high-level voiceprint feature sequence registered for the user.