A motorcycle rear box comprises a box body and a box cover. The utility model is characterized in that: an alarm which comprises a power supply, a key control switch a triggering switch and a loudspeaker is arranged inside the box body. The power supply, key control switch, triggering switch and loudspeaker are connected in series in order to form a circuit. After using the structure, because an alarm is provided inside the box body, the alarm will send out alarm automatically when the rear box is prized up illegally and thus it improves the motorcycle post box's performance of guarding against theft. Meanwhile, the utility model of motorcycle post box also has the advantages of simple structure, low cost and convenient installation, etc.