Abstract: The expansion stern is an important factor affecting the flatting trajectory and its stability of a trans-media vehicle during high speed water entry and turning flat process. In this paper, based on the fluid volume multiphase flow model and dynamic mesh technology, the coupling calculation method of multiphase flow field and trajectory of the trans-media supercavitating vehicle entering water at high speed is established. The accuracy and applicability of the numerical calculation method are verified by the experiments. Through the numerical simulation study on the high speed water entry and turning flat process of the trans-media vehicle, the influence of the expansion stern on the cavity development morphology, hydrodynamic characteristics and trajectory characteristics of the vehicle during the water entry and turning flat process is obtained, and the influence of the cone angle of expansion sterns on the flatting trajectory during high speed water entry is analyzed. The results show that when the vehicle without the expansion stern entering water and turning flat under the different preset rudder angles, the angle of attack increases continuously, eventually leading to the divergence of the flatting trajectory. After the vehicle with the expansion stern entering water, the recovery moment is formed when the expansion stern is wetted, and the stable flatting trajectory is obtained. The vehicles with different expansion stern cone angles (1.5°, 6°, 8°) have formed three different kinds of trajectory characteristics: stable planing, single-sided tail-slapping and double-sided tail-slapping, and all of them can achieve stable flatting trajectory. The principle of stable planing trajectory is the dynamic balance under the coupling effect of the preset rudder angle and expansion stern planing. This trajectory has the smallest comprehensive drag coefficient, the highest flatting efficiency and the smallest dynamic load, which is an ideal flatting trajectory form for the trans-media vehicle during high speed water entry.