Dileptons, as an electromagnetic probe, are crucial to study the properties of Quark-Gluon Plasma (QGP) created in heavy ion collisions. We calculated the invariant mass spectra and the anisotropic emission of thermal dielectrons from Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) energy $\sqrt{s_{NN}}=200$~GeV based on EPOS3. This approach provides a realistic (3+1)-dimensional event-by-event viscous hydrodynamic description of the expanding hot and dense matter with a very particular initial condition, and a large set of hadron data and direct photons (besides $v_{2}$ and $v_{3}$ !) can be successfully reproduced. Thermal dilepton emission from both QGP phase and the hadronic gas are considered, with the emission rates based on Lattice QCD and a vector meson model, respectively. We find that the computed invariant mass spectra (thermal contribution + STAR cocktail) can reproduce the measured ones from STAR at different centralities. Different from other model predictions, the obtained elliptic flow of thermal dileptons is larger than the STAR measurement referring to all dileptons. A clear centrality dependence of thermal dilepton occurs not only to elliptic flow $v_{2}$ but also to high orders for centrality in 0-60\%. At a given centrality, $v_{n}$ of thermal dileptons decreases neatly with $n$ for $2 \leq n \leq 5$.