<p>We conduct three-dimensional lattice simulations to study the density perturbation and gravitational waves (GWs) during the first-order phase transition (FOPT). We find that for phase transition strength <i>α</i> &gt; 1, the forward motion of bubble walls becomes the primary source, whereas for <i>α</i> &lt; 1, the dominant contribution to the density perturbation comes from the delay of vacuum decay. Additionally, the power spectrum of density perturbations generated by the phase transition exhibits a slope of <i>k</i>³ at small wavenumbers and <i>k</i>^−1.5 at large wavenumbers. Furthermore, we calculate the GW power spectra, which exhibit the slope of <i>k</i>³ at small wavenumbers and <i>k</i>⁻² at large wavenumbers. Our numerical simulations confirm that slow PTs can produce PBHs and provide predictions for the GW power spectrum, offering theoretical support for GW detection.</p>