The invention discloses a method and device for determining lost users, and relates to the technical field of the communication. Through the method disclosed by the embodiment of the application, thepossible lost users in the disaster can be timely determined when a target area is in large-scale disasters and like situations, and then the convenience is provided for rescue workers for determiningsearch targets and reducing the search range. The method comprises the following steps: acquiring user call data at the target area; determining a first user set according to the user call data, wherein the first user set comprises the users with service before a predetermined time node and the users without service after the predetermined time node in the target area; and determining the lost users according to the first user set. The method and device disclosed by the invention is applied to the rescue workers.