A multistage solution treatment process was applied for nickel-based single crystal superalloys, complemented by various aging durations and cooling rates. The microstructure was characterized by scanning electron microscopy (SEM) to observe the γ' phase. Additionally, phase field simulations were conducted to model the growth of γ' precipitates during aging and analyze their morphological evolution. The experimental results demonstrated that the multistage solution treatment effectively eliminated eutectic phases and carbides. Moreover, samples aged for 10 min exhibited larger and more rectangular γ' precipitates compared with those aged for 5 min. Notably, secondary γ' precipitates were observed in samples subjected to water cooling. Two indices for quantifying rectangularization were proposed and successfully applied. Based on the simulation results, lattice mismatch induced coherency stresses and elevated stress triaxiality along the <111> direction contributed to the rectangularization of the γ' phase.