The precipitation strengthening behavior and its effect on mechanical properties of Al-Mg-Si alloy during isothermal aging were investigated. In-situ synchrotron small angle X-ray scattering (SAXS) technique was used to characterize the nanoprecipitates and their structural parameters (size, volume fraction and number density) combined with transmission electron microscopy (TEM). The results show that rod-shaped β'''''''' precipitates grow preferentially along their longitudinal dimensions, but the radial dimension reaches a plateau after 5 h of aging. The growth of nanoprecipitates in size and volume fraction is quantitatively related to precipitation strengthening, as shown by predictions and measurements of relevant yield strength and hardness. SAXS provides more reliable input parameters for the Ashby-Orowan model, which improves the predictive accuracy and generalizability of the model. The study demonstrates that with increasing the aging time, the evolution of predictive mechanical properties of Al-Mg-Si alloy is dependent on the mean radius and volume fraction of β'''''''' precipitates.