The embodiment of the invention discloses a wireless local area network (WLAN) access authentication method and system based on World Wide Web. The WLAN access authentication method comprises the following steps: AC intercepts a DNS (domain name server) analysis request sent by a WLAN terminal and judges whether the WLAN terminal has passed user authentication or not; if the WLAN terminal does not pass the user authentication, the AC redirects the DNS analysis request to a local DNS which is isolated with a public network; the local DNS constructs a DNS response data packet pointing to an IP address of a portal server and returns to the AC; in response to receiving an HTTP request or an HTTPS request sent by an unauthorized WLAN terminal, a controller is accessed to redirect the HTTP request or the HTTPS request to the portal server; and the portal server sends a WEB authentication page to the WLAN terminal, acquires a user name and password input by the WLAN terminal by virtue of the WEB authentication page, and performs user authentication on the WLAN terminal according to the user name and the password. By adopting the WLAN access authentication method and system disclosed by the embodiment of the invention, the technical problem that the unauthorized WLAN terminal bypasses a Portal authentication process to perform illegal network access in the prior art can be solved.