The underwater launch stability of multi-medium vehicles is critically challenged by the dynamic evolution of shoulder and tail cavitation bubbles, which generate asymmetric loads and compromise water-exit success rates. To address this, we designed a novel unconstrained underwater launch platform integrated with 3D-printed modular vehicle models (flat/round heads) and multi-modal sensing systems (high-speed cameras at 16,000 fps + triaxial accelerometers), enabling quantitative visualization of cavitation dynamics under controlled cavitation numbers (σ = 0.62–2.51). Critical findings reveal: Cavitation number governs development asymmetry—lower σ compresses shoulder bubbles longitudinally (maximum L = 1.8D at σ = 0.62 vs. 1.2D at σ = 2.51), while higher σ promotes lateral expansion (W peaks at 0.82D). Collapse-induced instability mechanisms—pressure oscillations within bubbles trigger nonlinear collapses (30% higher collapse velocity at σ = 0.62), generating asymmetric forces that amplify yaw deviations by 15%–22%. Tail cavity dual-phase dynamics—at σ < 1.38, tail bubbles exhibit secondary detachment with umbrella-shaped collapse (velocity rebound ΔV = 1.2 m/s), whereas σ > 2.51 induces early necking at launch tubes, reducing effective thrust by 40%. These findings establish quantitative relationships between σ and bubble-mediated instability, providing design guidelines for cavitation-resistant vehicle profiles and adaptive launch control systems. The experimental framework offers a cost-effective alternative to prototype testing, reducing development costs by 60% through scalable 3D-printed models.