Leptonic and semi-leptonic D decays at BESIII contribute the most precise experimental measurement of |Vcs(d)| and form factor fD(s) in the world based on 2.93 fb−1 and 3.19 fb−1 data taken at center-of-mass energies s√=3.773 and 4.180~GeV, respectively. The largest samples at the mass threshold of the charmed hardons D(s) also provide chances to extract form factors of some semi-electronic decays for the first time and together with the semi-muonic decays we could understand lepton flavour universality better.