The present application relates to the field of 3D display technique, and provides a parallax device and a 3D display device. The parallax device includes a first grating unit and a second grating unit. The first grating unit includes first barriers and second barriers arranged alternately. The second grating unit includes third barriers that are in one-to-one correspondence with the first barriers. The third barriers are configured to prevent light transmitted through the first grating unit from entering and interfering with a viewing region.