In this work, the isothermal liquid-liquid equilibrium (LLE) and isobaric vapor-liquid equilibrium (VLE) of the methacrolein-methacrylic acid-water ternary mixture were systematically measured using an LLE cell and improved Rose-Williams still under atmospheric pressure. The thermodynamic consistency of LLE data and VLE data was tested by the Othmer-Tobias and Van Ness methods, respectively. The NRTL, UNIQUAC, and Wilson activity coefficient models have been conducted to correlate binary parameters in prediction phase equilibrium based on experimental data. The reliability of three models in equilibrium calculations has been analyzed in detail. All three models are in good agreement with the experimental data.