A keyword spotting system includes a decoder having a storage device and a decoding circuit. The storage device is used to store a log-likelihood table and a plurality of dynamic programming (DP) tables generated for recognition of a designated keyword. The decoding circuit is used to refer to features in one frame of an acoustic data input to calculate the log-likelihood table and refer to at least the log-likelihood table to adjust each of the DP tables when recognition of the designated keyword is not accepted yet, where the DP tables are reset by the decoding circuit at different frames of the acoustic data input, respectively.