Abstract: Objectives To investigate the influence of viscoelastic fluid injection on turbulence drag reduction and flow structure evolution in submersible vehicle boundary layers, providing new ideas for efficient drag reduction technologies in underwater vehicles. Methods Based on the SUBOFF submarine full-appendage geometric model, large eddy simulation (LES) combined with the dynamic sub-grid stress model was used to simulate and analyze the effects of the main rheological parameters of viscoelastic fluid (relaxation time λ = 0.01~0.1) and the incoming flow velocity (1~4 kn) on the drag characteristics, flow field structure and concentration diffusion behavior of viscoelastic fluid of the underwater vehicle. . Grid independence verification (11.58 million cells) and experimental validation (LES error <5%) ensured method reliability. Results Viscoelastic fluid significantly reduced friction drag when the inflow velocity reached ≥2 kn, achieving a maximum drag reduction rate of 33.47% (λ=0.01, 4 kn) and a total drag reduction rate of up to 29.89% (λ=0.1, 4 kn). At low speeds (1 kn), viscoelastic fluid formed a stable boundary layer attachment, while medium-high speeds (3–4 kn) induced turbulent vortices near the wall (e.g., Kármán streets behind the conning tower); increasing relaxation time (λ=0.1) suppressed low-speed vortex intensity but weakened its effect at higher speeds. At low speeds, concentration diffusion remained uniform, whereas diffusion range became restricted at speeds >3 kn; extending relaxation time (λ=0.1) enhanced low-speed diffusion capacity. Conclusions Viscoelastic fluid injection demonstrates active drag reduction potential at high speeds by reducing friction drag. However, coordinated optimization of relaxation time and inflow velocity is required to suppress pressure drag increments.