DTA,TG and DSC-TG analysis are used to study the pre-ignition reaction of B/Pb₃O₄ pyrotechnic delay compositions B1,B2 and B4.The reaction residues are determined by X-ray diffraction (XRD) analysis. The reaction mechanism is proved to be a solid-to-solid reaction which starts mainly at temperatures about 350 ℃ and 450 ℃. The chemical reaction formula in stoichiometry is proposed.