Abstract: This paper analyzes the four-way handshake protocol of WLAN's new generation security standard IEEE802.11 i.In order to solve the problem of DoS attack of forging unprotected message,an improved scheme is proposed based on message authentication with an algorithm SHA-256.The proposed scheme uses PMK to authenticate the first message of four-way handshake protocol.It introduces the high-security hash function SHA-256 to handle PMK and then uses the hash value of PMK to make an authentication of the first message.So it can distinguish and reject the forging message and ensure successful validation of the third message.Experiments show that under the same strength attack improved scheme reduces the AP's network load rate and increases the number of STA which successfully access to WLAN,so it can improve network performance.