A fluid reactor having a thin film with two-dimensionally distributed micro-resistor units includes one flow way, separating ribs, a thin film which is thinner than 100 micrometer, and a catalytic layer. There are multiple micro-resistor units, which is thinner than 60 micrometer each, set on the thin film. A bi-directional control unit is disposed to control these multiple micro-resistor units to shift to a desired mode selected from detecting electrical resistance, voltage, current, temperature, flow speed, and/or heat-up functions at different time. It can be applied to different kinds of fuel cells with different flow-way patterns. Its micro-resistor units have bi-directional functions. It can detect flow speed at a desired location in the flow way. It would not influence its original flow field and structure. Plus, it is easy to be installed.