We propose a new type of dispersion-flattened waveguide without a slot-assisted structure that can obtain an ultra-flat group velocity dispersion profile with five or six zero-dispersion wavelengths in the mid-infrared region. The dispersion profile becomes less sensitive to the waveguide dimensions due to the absence of the slot-assisted structure, making waveguide fabrication more friendly. The dispersion profile varies between -0.472 and 0.365ps/(nm · km) over a 2665 nm bandwidth from 2885 nm to 5550 nm with a flatness of 3183.99 nm² · km/ps. Two different combinations of materials are demonstrated for dispersion flattening of the proposed waveguide structures. We also provide design guidance for the proposed waveguide structures with other combinations of materials.