A method and a system for fast inspecting a vehicle based on a length measuring device, including: when a subject vehicle enters an inspection region, measuring a first length and a second length of the subject vehicle; determining whether the first length and the second length is respectively larger than or equal to a preset second length threshold; if so, determining whether a gap portion of the subject vehicle between a first portion and a second portion of the subject vehicle appears in a beam emitting region formed by a beam of radiation rays emitted by the system for fast inspecting a vehicle; and when the gap portion appears in the beam emitting region, emitting a beam of radiation rays of a first radiation dose to the subject vehicle according to the gap portion, wherein the subject vehicle moves with respect to the system for fast inspecting a vehicle.