Embodiments of the present invention provide a method and a device for sending data in a passive optical network. A central office processing device records a downstream superframe number of a downstream data frame. A loop delay between the central office processing device and a terminal processing device is obtained according to the recorded downstream superframe number in combination with an upstream superframe number and an upstream frame synchronization sign in an upstream channel-associated data frame sent by the terminal processing device, so that the central office processing device may, after subtracting the loop delay, accurately determine, by using cached bandwidth map information and an obtained equalization delay of the terminal processing device, a start time point and an end time point for sending an upstream service data frame to an optical line terminal.