The present disclosure provides a discovery method, a device and a system for an application layer traffic optimization server, which relate to communication field, so as to reduce the cross-domain traffic and meet the network optimization demand of a network ISP as an internet access point. The method for an application layer traffic optimization server, related to a terminal side, comprising: obtaining a domain name of a proxy server for a terminal data layer to access internet; obtaining, according to a query name constructed using the domain name of the proxy server, an address of an application layer traffic optimization ALTO server related to the proxy server from a domain name server DNS; and communicating with the ALTO server according to the address of the ALTO server. Embodiments of the present disclosure are mainly applied to the communication between the terminal and the ALTO server.