Due to the small object distance and large divergence angle of light source in the existing laboratory microscope, it is impossible to observe the cells in the culture tank in real time. A micro system for external observation of cell culture tank with different capacity is designed. The working distance of the system is 45mm, NA=0.19, and the image resolution δ=10.8um. The design results show that the meridian coincides well with the Hu vector line, which is close to the theoretical diffraction limit. At the space frequency of 88lp / mm, the MTF of each field of view is above 0.05, which has high resolution. The imaging size of each field of view is smaller than the Airy spot radius, and the imaging quality is good.