The invention provides a vehicle-mounted data unit (10), a method, engineering machinery and a system of cantilever crane vibration monitoring. The vehicle-mounted data unit (10) comprises a vibration detector (1), a controller (2) and a storage device (3), wherein the vibration detector (1) is used for detecting cantilever crane vibration and outputting vibration signals characterizing the cantilever crane vibration in real time, the controller (2) is used for receiving the vibration signals output by the vibration detector (1) to generate vibration data and to compare vibration intensity obtained according to the vibration signals with a preset vibration triggering threshold valve, when the vibration intensity is larger than the vibration triggering threshold value, the controller (2) transmits the vibration data to the storage device (3), and the storage device (3) is used for storing the vibration data received. Thus, when a vibration problem happens, even through a technician is not on site, first-hand on-site vibration data can be recorded, and the accident that timely data recording can not achieved due to the transfer of a pump truck can not happen. Moreover, the vibration data is stored only when the vibration is too strong, the storage times of the storage device are reduced, and the service life of hardware is prolonged.