This invention involves an online cell microscopic observation instrument for biochemical reactor, including the main instrument of the observation, the objective lens, the observation window incident, sampling devices, external light source system. Its characteristics are: reflectance prism is installed between external light source and lens system. There is a reflecting mirror behind the objective lens and in the front of entrance window. Reflecting prism is placed in the front of annular aperture films. the upper side of the reflecting prism has a home or area array CCD image sensor. Sampling device includes sampling block, elastic components, a mobile device. The elastic element is installed between sampling block and the drive shaft of mobile devices. the gab between the front face of sampling block and observation entrance window develops the sampling pool. The external light source system of this invention can easily replace wavelength through different filter replacement; dark field of view at the same time lighting can improve resolution of observation instrument for the cells into the contrast of the image and resolution; and it is simple in structure, easy-to-use, manufacturing costs low.