ing at the technical challenge that it is difficult to realize the on-site detection of the particle size of the underground injection water for the water-drive oilfield development, we present a design concept of an optical system based on the optical scattering principle for the on-site injection water particle size detection. Using a laser beam with a wavelength of 640 nm and a 4× collimated beam expander system, we obtain an 8 mm illumination aperture. Using pupil matching and aberration balance, we achieve the optimal design of a compact optical system with a diameter of 42 mm, which meets the particle size detection requirements in the 1--100 μm range and has the ability to realize the real time underground on-site detection. The focal length of the optical system is 50 mm, the field of view is 17°, the aperture is 8 mm, and the spot radius within the field of view is less than 6 μm, which can effectively avoid the interference of non-scattered light on the energy of the photosensitive ring. The detection of standard particle samples via prototype shows that the test accuracy is less than 5%, which meets the design requirements and thus verifies the feasibility and rationality of the designed system.