An in-situ replaceable LED car headlight and a design method, comprising a lamp body shell, a light source assembly provided with LED lamps, a driving assembly, and a fan, wherein the light source assembly is arranged at a head of the lamp body shell, and the driving assembly is arranged at a tail of the lamp body shell; a middle part of the lamp body shell is provided with a central installation part for accommodating the fan, and the fan is installed between the light source assembly and the driving assembly; the driving assembly is electrically connected with the fan by adopting a flexible board connecting wire. A structure in which the fan is arranged in the central position is adopted, so that the fan can be placed in the part with the largest internal space of the lamp body within the range limited by laws and regulations.