To explore the unsteady flow characteristics and evolution mechanisms of the stern jet of a pump‑jet propulsor，based on the numerical simulation results of the DES model，the dynamic mode decomposition method was used to decompose and reconstruct the stern jet flow field of the pump‑jet under different cruising conditions.By comparing the numerical simulation with experimental data，it was found that the average error of thrust performance was 6.6%，verifying the effectiveness of the model.The results indicate that the stern jet flow field of the pump‑jet can be decomposed into flow features with different energies and frequencies.The first five modes account for more than 99.9% of the total energy.The fundamental mode with zero frequency represents the steady‑state structure of the unsteady flow field，reflecting the steady‑state flow field characteristics of the pump‑jet wake under different speeds and rotor rotations.Other major modes reveal the influence of the development process of high‑energy fluid masses in the wake on the wake flow bundle.Through the dynamic mode decomposition method，a low‑dimensional approximation of the high‑dimensional multi‑scale flow field can be effectively achieved，with reconstruction errors of 4.5% （low speed），5.3% （medium speed），and 4.4% （high speed），reflecting the unsteady characteristics of the complex flow field.As the speed increases，the energy becomes more concentrated in a few dominant modes.The proportion of the first‑order mode in the total energy gradually increases，approximately 95.2% at low speed，97.6% at medium speed，and 98.9% at high speed.The period of the modal coefficients increases，the flow field stability is enhanced，the wake flow bundle fluctuation of the first‑order mode decreases，and the velocity variation becomes more uniform.This study can provide a theoretical basis for the structural optimization design of pump‑jet propulsors and wake field stability control.