Periodic first-principles calculations have been performed to study the effects of electric field on the geometric and electronic structures and decomposition mechanism of crystalline lead azide (alpha-Pb(N-3)(2)). The results show that the influence of external electric field on the crystal structure is anisotropic owing to the different crystal packing along three crystallographic directions. The applied field causes a little change in unit cell volume, and the crystal symmetry remains unchanged, indicating that lead azide does not undergo a phase transition at an applied field smaller than 8.017 V/nm. The electric field effects on the ionic Pb-N bonds are stronger than those on the covalent N-N bonds because more effective charges reside on the atoms in the former. Moreover, the applied field across the crystal develops instabilities. The Franz-Keldysh effect yields larger influence on the band gap than the structural change induced by applied field. At the field higher than 4.453 V/nm, lead azide has metallic properties. Additionally, lead azide is more sensitive to external electric field than lead styphnate. Although the electric field redistributes the electron density of the frontier molecular orbitals, the decomposition mechanism of lead azide in the presence and absence of the field is similar. Finally, the electric-field-induced decomposition of lead azide produces N-2 and metallic lead.