A fluid channel structure for polar plate of fuel cell includes a plurality of parallel passages provided in a reaction area of a polar plate, two through holes of fluid inlet and outlet provided on the polar plate, a plurality of communicating channels connecting the through holes to two ends of the passages, and a plurality of tunnels formed between the communicating channels and each of the through holes. The tunnels are formed by closing a top cover onto a locating zone formed on the polar plate near the through hole, so that slots formed at a lower side of the top cover are aligned with horizontal groove sections and vertical step sections formed in the locating zone to form the tunnels. The top cover bears a relatively large fluid pressure, enabling the fluid channel structure to transfer sufficient reactant fluid to the polar plate without the risk of leaking.