The invention discloses user equipment, and a method and a device for processing message mailbox fault under an AMP (Asymmetric Multi-Processing) system. The AMP system performs inter-core communication via the message mailbox. The processing method comprises steps: a second core determines an abnormality happens to a first core, and the second core stops adding a task for sending a message to the first core to a second core INPUT BOX, wherein the first core is a core to which the abnormality happens, and the second core is a core working normally; data in the second core INPUT BOX are cleared; and the second core recovers the ability for inter-core communication via the message mailbox. After the second core determines an abnormality happens to the first core, the data in the second core INPUT BOX are cleared, the second core stops adding a task for sending a message to the first core to the second core INPUT BOX, and thus, before a core to which the abnormality happens is back to normal, inter-core communication is firstly recovered between two cores working normally, the robustness of the system is enhanced, and the scheme is realized in a software mode, and an extra hardware cost does not need to be added.