By means of the in-situ Fourier transformation infrared spectroscopy(FTIR), the properties of BBr3/benzene solution, which is usually used as the reactant and solution to synthesize BN by benzene-thermal method, have been investigated. The results show that there are some side reactions between BBr3 and benzene: (1) BBr3 as an electron-deficient molecule reacts with benzene at room temperature; (2) below 100 °C, substitution of Br atom for H atom of benzene(ring―H) dominates in BBr3/benzene solution; (3) cracking of benzene ring occurs at a temperature above 100 °C; (4) decomposition of benzene molecules and formation of long-chain aliphatic compounds feature the spectra of BBr3/benzene solution collected at above 160 °C. They are unfavor for BN to form when BBr3 is excessive in the synthesis of BN by benzene-thermal route. On the basis of the experimental results, a coordination reaction mechanism via a η2-C6H6 binding mode in BBr3/benzene solution is suggested. The means of the in-situ Fourier transformation infrared spectroscopy (FTIR), the properties of BBr3 / benzene solution, which is usually used as the reactant and solution to synthesize BN by benzene-thermal method, have been investigated. The results show that there (2) below 100 ° C, substitution of Br atom for H atom of benzene (ring-H) dominates in (1) BBr3 as an electron-deficient molecule reacts with benzene at room temperature; (3) cracking of benzene ring occurs at a temperature above 100 ° C; (4) decomposition of benzene molecules and formation of long-chain aliphatic compounds feature the spectra of BBr3 / benzene solution collected above at 160 ° C . They are unfavor for BN to form when BBr3 is excessive in the synthesis of BN by benzene-thermal route. On the basis of the experimental results, a coordination reaction mechanism via a η2-C6H6 binding mode in BBr3 / benzene solution is suggested.