The invention discloses an encryption and decryption system based on the medium security protection of an IPTV platform. On a platform end in the method, each sub-block of a media file is firstly carried out with MD5 operation, then a section of information is combined by the MD5 values obtained from the sub-blocks, the information is carried out with MD5 operation again, an obtained result is placed in a playlist, and finally, the playlist is encrypted. On a terminal, firstly, the playlist is decrypted, MD5 values of information abstracts of the media file, which are obtained from the playlist, are compared with the MD5 of information abstracts combined by all the sub-blocks of an operation file; if the two MD5 values are not identical, the play is prohibited; if the two MD5 values are identical, the MD5 of each sub-block is compared, if identical, the information is placed in a buffer area to by played; and if else, the play is stopped. The method has high speed and high safety factor.