The solubility of 3,4,5-trimethoxybenzaldehyde (TMBA) in five kinds of aqueous mixed solvents including water + (ethanol, n-propanol, i-propanol, acetone, or N,N-dimethylformamide (DMF)) was determined by a gravimetric method. The temperature range was 278.15 to 313.15 K. The solubility of TMBA is positively correlated with temperature in all the selected aqueous mixed systems. The results show that the solubility of TMBA in the solvents of water + alcohol is lower than that in water + DMF or water + acetone. The maximum solubility (x 1 = 0.3669 mol•mol −1 in the solvent of water + DMF, T = 313.15 K, and x 2 = 0.9) is about 1800 times higher than the minimum solubility (x 1 = 0.0002010 mol•mol −1 in the solvent of water + ethanol, T = 278.15 K, and x 2 = 0.1). There is a cosolvency phenomenon in the water + alcohol solvent systems. Furthermore, thermodynamic models including the modified Jouyban−Acree model, Sun model, nonrandom two-liquid (NRTL) model, and Wilson model were used in this paper. Relative average deviation and root-mean-square deviation values are not larger than 0.0783 (Sun model, water + DMF) and 0.002625 (NRTL model, water + acetone). The Wilson model and the NRTL model fitted better, and all the selected models provided satisfactory correlation results.