High pressure structural behavior of Bi2Fe4O9 has been studied by in situ angular-dispersive X-ray diffraction (ADXD) measurements up to 51.3 GPa. Two phase transitions have been observed at 7.6 and 22.6 GPa, respectively. A second high pressure structure (HP2) involving the tripling of lattice parameter c has been identified. An unusual amorphization occurs after releasing pressure. The high pressure phase transitions can be understood in terms of the increase in the coordination number of Fe3+ ion. The depressurization amorphization results from the appearance of the metastable HP2 and its collapse after releasing pressure. The results extend our understanding of pressure-induced amorphization.