The invention relates to a P-N-B high-flame-retarding flexible epoxy resin composition, being a powder, and a preparation method thereof. The composition contains three flame-retarding elements of P,N and B, which are all located on a molecular chain, and contains a phosphine oxide structure and a flexible chain segment, so that when being applied to electronic packaging materials, the composition greatly improves the flame-retarding effect, flame-retarding durability and cold/heat impact resistance of the electronic packaging materials, and also optimizes the heat resistance, chemical resistance and damp-proof effect of the electronic packaging materials.