The invention discloses a method for the multistage sampling of a floating-roof oil tank. The method comprises the following steps of: designing an upper part sampling pipe, an upper middle part sampling pipe, a middle part sampling pipe, a lower middle part sampling pipe and a lower part sampling pipe from the upper part, the upper middle part, the middle part, the lower middle part and the lower part of the circumferential side of a central drainage pipe of the floating-roof oil tank to the outside of the lower side of the same circumferential side of the floating-roof oil tank; connecting the distal ends of the sampling pipes in parallel and then connecting with an inlet of a gear pump, and respectively designing a sampling control valve at the pipelines before being connected in parallel; connecting an outlet of the gear pump with the lower part of the floating-roof oil tank by an oil return pipe, and designing a gear pump outlet control valve and a sampling valve at the front part and the back part of the oil return pipe; and opening a gear pump inlet control valve and the gear pump outlet control valve of the upper part sampling pipe when sampling oil of the upper part of the floating-roof oil tank, starting the gear pump to firstly drain the dead oil in the sampling pipes into the floating-roof oil tank through the oil return pipe, and then opening the sampling valves to sample, wherein the sampling process of each part is same. The method of the invention is convenient to the multistage sampling of the floating-roof oil tank and also saves time and labor.