A single-mode square photonic crystal fiber (S-PCF) for the wide-band dispersion compensation at the wavelength interval 1340 to 1640 nm over the telecommunication windows is represented. In the numerical analysis of several guiding properties of the photonic crystal fiber (PCF), a finite-element method has been applied over the entire wavelength band. According to the investigation, the proposed PCF is strongly single-mode, has an ultra-high negative dispersion coefficient of about −1732.10 ps/(nm· km) at an operating wavelength of 1550 nm, and is perfectly applicable to optical transmission systems. The single-mode behavior of the S-PCF is examined by applying the V parameter. By the exquisite dispersion analysis, this fiber is more robust for obtaining the ultra-high negative dispersion, which gains more promiscuous as compared to the prior best results. For high-speed transmission systems, such fibers are effective as compensating the dispersion and maintaining a single polarization, as well as in fiber loop mirrors and many other applications like the four-wave mixing.