A signal classifying method in speech or music coding, comprising: obtaining a spectrum fluctuation parameter of a current signal frame, being either a foreground frame or background frame; buffering the spectrum fluctuation parameter of the current signal frame in a first buffer array if the current signal frame is a foreground frame; if the current signal frame falls within a first number of initial signal frames, setting a spectrum fluctuation variance of the current signal frame to a specific value and buffering the spectrum fluctuation variance of the current signal frame in a second buffer array; otherwise, obtaining the spectrum fluctuation variance of the current signal frame according to spectrum fluctuation parameters of all signal frames buffered in the first buffer array and buffering the spectrum fluctuation variance of the current signal frame in the second buffer array; and calculating a ratio of signal frames whose spectrum fluctuation variance is above or equal to a first threshold to all signal frames buffered in the second buffer array, and determining the current signal frame as a speech frame if the ratio is above or equal to a second threshold or determining the current signal frame as a music frame if the ratio is below the second threshold; characterized by that the first threshold is a first adaptive threshold, and the first adaptive threshold is obtained according to a Modified Sub-band Signal Noise Ratio (MSSNR) or a Signal-to-Noise Ratio; wherein the step of obtaining the first adaptive threshold according to the MSSNR comprises: updating a maximal value of the MSSNR according to the current signal frame; determining a threshold of the MSSNR according to the updated maximal value of the MSSNR; obtaining the number of frames whose MSSNR is above the MSSNR threshold and number of frames whose MSSNR is below or equal to the MSSNR threshold among a certain number of frames inclusive of the current signal frame; calculating a difference measure between the number of frames whose MSSNR is above the MSSNR threshold and the number of frames whose MSSNR is below or equal to the MSSNR threshold, and obtaining the first adaptive threshold according to the difference measure. obtaining a spectrum fluctuation parameter of a current signal frame, being either a foreground frame or background frame; buffering the spectrum fluctuation parameter of the current signal frame in a first buffer array if the current signal frame is a foreground frame; if the current signal frame falls within a first number of initial signal frames, setting a spectrum fluctuation variance of the current signal frame to a specific value and buffering the spectrum fluctuation variance of the current signal frame in a second buffer array; otherwise, obtaining the spectrum fluctuation variance of the current signal frame according to spectrum fluctuation parameters of all signal frames buffered in the first buffer array and buffering the spectrum fluctuation variance of the current signal frame in the second buffer array; and calculating a ratio of signal frames whose spectrum fluctuation variance is above or equal to a first threshold to all signal frames buffered in the second buffer array, and determining the current signal frame as a speech frame if the ratio is above or equal to a second threshold or determining the current signal frame as a music frame if the ratio is below the second threshold;