The invention discloses a laser offset monitoring device and method for a main tower of a large-span bridge. The monitoring device comprises an adjustment plate, a laser range finder and a cloud computing data processing server, the adjustment plate is arranged in a center line position of the top of the bridge tower, the surface of the plate is vertical to the side surface of the bridge tower, the laser range finder is arranged in a center line position of the bottom of the bridge tower, an included angle between the axis of the laser range finder and the plate surface is theta, the laser range finder is connected with a wireless data emitter, and the cloud computing data processing server receives a signal from a wireless data emitter and calculates the left-right offset of the bridge tower according to the signal. Thus, the adjustment plate converts the vertical displacement value measured by the laser range finder into the horizontal (left-right) offset value of the bridge tower, the angle of the adjustment plate is adjusted to provide a precision coefficient for the measuring value, and the measuring sensitivity and precision are improved.