BESIII has collected 2.93 fb$^{-1}$ and 6.32 fb$^{-1}$ of $e^+ e^-$ collision data samples at the center-of-mass energies 3.773 and 4.178-4.226 GeV, respectively. We report recent measurements of the (semi-) leptonic decays $D_{s} \to \ell^+ \nu_{\ell}~(\ell=\mu, \tau)$ and $D_{(s)} \to X \ell^+ \nu_{\ell}$ [$X=\eta, K_{1}(1270)$ or denotes generic decays, and $\ell=e, \mu$]. The decay constant $f_{D_s^+}$, the semileptonic form factor $f^{\eta}_{+}(0)$ and the CKM matrix elements $|V_{cd(s)}|$ are determined precisely. These results are important to verify the theoretical calculations of $f_{D_{s}^+}$ and $f^{\eta}_{+}(0)$, and the unitarity of the CKM matrix. Precision tests of lepton-flavor universality with (semi-) leptonic $D_{(s)}$ decays are also performed.