The invention discloses a gas chromatograph for analysing the total-hydrocarbon constituents of gasoline and diesel catalytic cracking products. The gas chromatograph comprises a six-way valve, a twelve-way valve, three chromatographic columns, two quantification tubes, two three-way stop valves and an FID (flame ionization detector), wherein C1-C5 light hydrocarbon constituents and C6<+> high-carbon hydrocarbon constituents in the gasoline and diesel catalytic cracking products are measured simultaneously by a valve and column switching technology. The gas chromatograph disclosed by the invention can be applied to research of oil refineries, and universities and colleges on a catalytic cracking reaction mechanism, and is convenient to operate, high in practicability, simple in structure and low in cost.