The wide applications and continuously developing manufacturing methods of heat-treatable aluminum alloys require accurate and efficient precipitate hardening models to predict the microstructure behaviors response to the aging process. The current article proposed a modified precipitate hardening model of A356 aluminum alloy used for engine block. In terms of the microstructure parameters of β” as the primary strengthening phase, the relative volume fraction is studied and modified because of the phase transformation in aging process. The proposed model successfully predicts the yield strength at underaged aging process in the temperature range of 150°C∼170°C after considering the impact of coexist precipitate phases such as β’ phase. After the modification, the deviation between the yield strengths at low temperature and high temperature was greatly eliminated. This successful modification draws attention to the precipitate phase volume fraction in the research of the precipitate hardening model. The composition of the precipitate phase and interactive transformation when altering the aging process parameters has a significant impact on the yield strength behaviors in the aging process.