The invention discloses an online analysis pre-processing and off-line analysis integrated liquid chromatograph and a use method thereof. The chromatograph comprises an autosampler, two chromatographic pumps, three six-way valves, an online purification column, a chromatographic separation column, an ultraviolet detector and a fluorescence detector. The chromatograph can be switched between off-line analysis and online pre-processing analysis according to use needs. When the chromatograph is used for online pre-processing analysis, the sample can be easily extracted and directly detected without pre-processing. The detection process is greatly shortened. Manual operation is reduced, and the analysis cost is reduced. When off-line analysis is carried out, switching is directly carried out without changing a system. The use efficiency and the analysis ability of the instrument are effectively improved.