Photonic bandgap fibers （PBFs） have advantages that traditional fiber cannot compare for its’ unique structural form， transmission medium， and light guide mechanism. PBFs is an ideal choice for fiber-optic gyroscope （FOG） in the future. However， the rough inner surface of PBFs’ core result in strong backscatter secondary wave， which results in additional non-reciprocal errors of photonic bandgap fiber-optic gyroscope （PBFOG）. In order to quantitatively analyze the intensity of backscatter secondary wave of PBFs， a simple backscatter secondary wave model of PBFs was established based on electric dipole radiation theory. Power spectral density of inner surface of PBFs’ core was obtained by focused ion beam processing and atomic force microscopy， and the theoretical backscattering coefficient of PBF （HC-1550-02） can be calculated as 2.61×10^-9/mm. The measured backscattering coefficient of PBF （HC-1550-02） is ~1.82×10^-9/mm by optical frequency domain back-reflection scatter instrument， which primarily verify the correctness of backscatter secondary wave model， it lays a foundation for the research of suppression technology of backscatter secondary wave.