The SMS4 is the first commercial block cipher published by Chinese government. It's a 32-round block cipher encrypted by 128-bit keys. By analyzing the changes of the difference between input and output pairs in each round, this paper presents a new impossible differential path of the 14- round SMS4. Using this path, a new method is submitted to crypt analyze an 18-round SMS4. The time complexity of the attack is 2^117:06 partial encryptions.