An optical detecting device has gas emission and pressure reduction function and includes a holder, a light penetrating component, a light detecting component, a hole structure, and a. The light penetrating component is disposed on the holder to form an accommodating space whereinside the light detecting component is disposed. The hole structure is formed on the holder to connect with the accommodating space. The waterproofing and ventilating component is disposed on the holder and covers the hole structure to prevent liquid from leaking into the accommodating space via the hole structure. While an inner gas pressure of the accommodating space is decreased, part of the gas is exhausted from the accommodating space via the hole structure and the waterproofing and ventilating component.