A flow passage design for multi-reaction biological detection includes a first temporary tank, a second temporary tank, a first microchannel, and a second microchannel. The first temporary tank is configured to temporarily store a first liquid in an initial state. The second temporary tank is configured to temporarily store a second liquid in the initial state. The first microchannel is located upstream of the first temporary tank. The first microchannel has an outlet end and an inlet end, respectively connecting to the first temporary tank and the second temporary tank. The second microchannel is located downstream of the first temporary tank and connects to the first temporary tank. In the initial state, a portion of the first liquid enters the second microchannel, the outlet end of the first microchannel is covered by the first liquid, and the inlet end of the first microchannel is covered by the second liquid.