The excellent performance of the deep eutectic solvent (DES) is consistent with the 12 Principles of Green Chemistry. It is of importance to study the DES’s structural characteristics, thermophysical properties and physicochemical properties, especially the DES’s solid–liquid-equilibrium phase diagrams in the entire mole fraction. In this work, ten DES were prepared using tetrabutylammonium bromide as the hydrogen bond acceptor, and octanoic acid/hexanoic acid as the hydrogen bond donors. The molar ratio ranged from 1:1.25 to 1:4. Then, the solid–liquid-equilibrium phase diagrams were obtained. The eutectics and the corresponding eutectic points were discovered. The nuclear magnetic resonance spectra and the Fourier transform infrared spectra were applied to analyze the interactions in DES. The densities and viscosities of the prepared DES were measured. With the optimized Lydersen-Joback-Reid method combined with Lee-Kesler rules, the critical properties were predicted. And then the results were verified through a density prediction model. The average absolute relative deviation (AARD) is better than 1.38%. The mass-connectivity-index-based model was introduced for density prediction with AARD of less than 0.17%. For the viscosity, the Arrhenius equation was compared with an equation considering the molar mass. The AARD of them were less than 2.68 % and 0.69 %, respectively.