Disclosed are an error code conversion method and system, a Point-to-Point Protocol over Ethernet (PPPoE) server and a mobile terminal, and the method includes: a PPPoE server parses a PPPoE error code generated during a dialup process, converts the PPPoE error code to an extended error code according to a PPPoE error code conversion rule table preconfigured on the PPPoE server and transmits the extended error code to a mobile terminal; the mobile terminal receives the extended error code, interprets the extended error code into a plain text according to a PPPoE error code conversion rule table configured on the mobile terminal in advance and displays the plain text to a user. The present disclosure improves capability of a PPPoE server and a terminal for processing errors, thus making it convenient for the mobile terminal and an operator to control a PPPoE dialup process and for a user to surf the Internet through WiFi by rapidly using PPPoE dialup functionality.