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• We develop a combined geodynamic, mineralogical and seismic modeling approach. • Thermal structure of the stagnant slab in the MTZ beneath SE China is constrained. • The morphology of the slab indicates that it underwent little deformation. The structure of Earth's mantle transition zone (MTZ) contains important information about how oceanic plates are recycled into the mantle. However, morphologies of stagnant slab in the MTZ inferred from seismic studies remain controversial. Here, we conduct combined geodynamic, mineral physical and seismologic modeling to infer fine features of the stagnant slab in the MTZ beneath southeastern China. This method applies mineral physics modeling in combination with geodynamically consistent thermal anomalies to predict seismic velocity structures in the MTZ that are used in triplication waveform modeling. The synthetic waveforms are systematically compared with observed data using a grid search optimization. Our best-fit model reveals a stagnant slab lying at the bottom of the MTZ with a peak temperature reduction of 350 ± 80 K and a thickness of 85 ± 20 km, indicating that it underwent little deformation. By joint modeling of the mineral physics and thermal history of the slab, our approach improves estimates of both the thickness and temperature of the stagnant slab and provides a better understanding of the paleo subduction system beneath southeastern China. [ABSTRACT FROM AUTHOR]