The perturbation in a fluid field can be captured by a visualized imaging system. In an optical schlieren system, we theoretically and experimentally compared the images of three-dimensional perturbation in the fluid field. The variations of fluid density generated by three-dimensional ultrasonic field are different in the direction of light propagation. As a result, after the light passes through the density-changing fluid field, the images cannot truly reflect the perturbation in the acoustic field due to the phase accumulation. Focusing schlieren systems with parallel line grids also cannot image the three-dimensional ultrasonic perturbation in the fluid field because of their multiple cutoffs. For this reason, we proposed a modified focusing schlieren system with annulus source and cutoff grids to obtain the information about three-dimensional perturbation. The images obtained by the optimized system achieve the same characteristics with the ultrasonic perturbation in the actual fluid field. In combination with image reconstruction techniques, this system supports the reconstruction of complex three-dimensional perturbation in the fluid field.