Hardware accelerator for compressed Long Short Term Memory (LSTM) is disclosed. The accelerator comprise a sparse matrix-vector multiplication module for performing multiplication operation between all sparse matrices in the LSTM and vectors to sequentially obtain a plurality of sparse matrix-vector multiplication results. A addition tree module are also included for accumulating a plurality of said sparse matrix multiplication results to obtain an accumulated result. And a non-linear operation module passes the accumulated results through an activation function to generate non-linear operation result. That is, the present accelerator adopts pipeline design to overlap the time of data transfer and computation for compressed LSTM.