The strong-isospin violation in psi(3770) -> J/psi +pi(0) via intermediate D meson loops is investigated in an effective Lagrangian approach. In this process, there is only one D-meson loop contributing to the absorptive part, and the uncertainties due to the introduction of form factors can be minimized. With the help of QCD spectral sum rules (QSSR), we extract the J/psi DD* form factor as an implement from the first principle of QCD. The DD* pi(0) form factor can be well determined from the experimental data for D -> pi l nu. The exploration of the dispersion relation suggests the dominance of the dispersive part via the intermediate D meson loops even below the open charm threshold. This investigation could provide further insights into the puzzling question on the mechanisms for psi(3770) -> non-D (D) over bar transitions.