Atmospheric aerosols have an important impact on the global environment and human health. Secondary aerosols generated in the troposphere are one of the main sources of atmospheric aerosols. Its formation is generally divided into two stages, the formation of critical nuclear and the following rapid growth process. Phthalic acid and sulfuric acid are common nucleation precursor in the atmosphere, however, their nucleation mechanism is still seldom studied. The structure and thermodynamic parameters of the cluster are obtained by high-precision quantitative calculation. The evaporation rate analysis shows that compared to sulfate clusters, the heterodimer formed by phthalic acid and sulfuric acid is more stable, that is, the addition of phthalic acid greatly reduces the evaporation rate of the sulfuric acid cluster, which indicates that phthalic acid has the effect of promoting nucleation of sulfuric acid. Optical property analysis shows that the cluster size is positively correlated with Rayleigh scattering and isotropic average polarizability. Moreover, infrared spectrum analysis also confirms the presence of hydrogen bonds in the heterodimer, which is in favor of the formation of stable nucleation clusters.