Nickel-base single crystal superalloy is frequently used for the turbine blades of aero-engine. The alloying element segregation during its solidification process leads to the heterogeneity in microstructure, which could be fatal to the high temperature mechanical property. DD5 is one of the second generation of domestic single crystal superalloy. A cylinder specimen was withdrawn by a rate of 3mm/min. Microstructure observation and thermodynamic calculation reveals the cast microstructure includes γ, γ' and carbide. Electron probe microanalysis was employed for the measurement of alloying element segregation across the dendrites and interdendrites, indicating that Re and W tends to segregate into dendrites while Al and Ta concentration is higher in interdendritic regions. In particular, the carbide precipitation in in-terdendritic region is proven to be Ta enriched, which is supposed to be MC type carbide. On this basis, phase field simulation on the microstructure evolution and alloying element segregation during solidification is performed via Mi-cress software. The simulated microstructure is quite similar to the experimental observation, and the alloying element distribution is in accordance with EPMA measurement.