A method and an apparatus for Voice Activity Detection (VAD) and an encoder are provided. The method for VAD includes: acquiring a fluctuant feature value of a background noise when an input signal is the background noise, in which the fluctuant feature value is used to represent fluctuation of the background noise; performing adaptive adjustment on a VAD decision criterion related parameter according to the fluctuant feature value; and performing VAD decision on the input signal by using the decision criterion related parameter on which the adaptive adjustment is performed. The method, the apparatus, and the encoder can be adaptive to fluctuation of the background noise to perform VAD decision, so as to enhance the VAD decision performance, save limited channel bandwidth resources, and use the channel bandwidth efficiently.