In this talk we present the first complete high-precision results for the top-quark $t$ decay width $\Gamma_t$, $W$-helicity fractions and semi-inclusive distributions for the $t$-decay process to the third order in the strong coupling constant $\alpha_s$. Our investigation reveals that the pure $\mathcal{O}(\alpha_s^3)$ correction decreases $\Gamma_t$ by $0.8\%$ of the previous $\mathcal{O}(\alpha_s^2)$ result, which exceeds the error estimated by the conventional scale-variation. With this critical piece of pertubative correction incorporated, our to-date most precise theoretical prediction meets the request by future high-energy colliders.