In this study, magnesium hydroxide nanoflakes (MHF) were synthesized using poly(ethylene glycol) (PEG 4000) surfactant by exploiting chemical precipitation method from an impure brine enriched by large amounts of CaCl2 for the first time. Synthesized samples were characterized by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR) and UV–Vis absorption spectroscopy. The effect of PEG on structural and morphological characteristics of synthesized samples, as well as the formation mechanism of MH nanostructures from the brine in the presence or absence of PEG (0, 5 and 10 mL), were precisely investigated. Obtained results implied the positive effect of the used surfactant on the uniformity of the morphology, the agglomeration behavior of MH nanoplates, and the decrease of the crystallite size (from 25.26 to 19.35 nm) of synthesized nanoparticles from the brine, which the optimum amount of this surfactant was 5 mL. In addition, UV–Vis spectroscopy results and investigation of optical properties showed that the presence of PEG led to an increasing in the value of the optical bandgap energy of MHF from 4.7 to 5 eV, indicating their ability to be used in optoelectronic nano-devices.