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; obtaining a plurality of high-level voiceprint features by tuning the first-level DNN model based on labeled speech data, the labeled speech data containing speech samples with respective speaker labels, and the tuning producing a second-level DNN model specifying the plurality of high-level voiceprint features; based on 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, the speaker verification comprising: receiving, from the user, a test speech sample; obtaining a second high-level voiceprint feature sequence based on the test speech sample using the first-level DNN model and the second-level DNN model in sequence; determining a distance between the second high-level voiceprint feature sequence and the first high-level voiceprint feature sequence registered for the user; and in accordance with a determination that the distance between the second high-level voiceprint feature sequence and the first high-level voiceprint feature sequence is less than a preset threshold, automatically, without user intervention, verifying the identity of the user.