A symmetric encryption/decryption method includes the steps of selecting a diffused mechanism, and the diffused mechanism includes at least one selected from a shift point, a block and a frame; obtaining a plurality of bits required for a cipher by the diffused mechanism and the element number of each dimension of a plaintext; carrying out at least one diffused operation for the plaintext; repeating the foregoing steps to achieve the effect of encrypting the plaintext. Since the sum of the encryption diffused times and the decryption diffused times equals to the diffused cycle, the cipher can be read and at least one dimensional diffused operation of the ciphertext can be carried out, and thus achieving the effect of decrypting the ciphertext.