【Objective】 Manganese (Mn) doping may affect iron oxide in structure and property, and then influence its performance in adsorption, catalysis and so on, especially its utilization in soils and environment. However, it is still little known about the detailed influence of Mn²⁺ doping in microstructure owing to complex hydrogen bonding and similar chemical bonding of the iron oxide crystallines. 【Method】 In this study, samples of crystalline iron oxides (CIO) and samples of Mn²⁺ doped CIO different in molar ratio (R) (CIO-Mn_x, x=0.1, 0.2, 0.3 and 0.5, separately) were prepared out of goethite and hematite. Influences of Mn²⁺ doping on iron oxide in crystal structure and morphology were analyzed with the aid of XRD and TEM. Peak fitting was performed of the two wave number ranges, high (3 000-3 700 cm^-1) and low (450-750 cm^-1) of the FT-IR graphs of the samples, and changes in hydroxyl functional group and crystalline chemical bonding of Mn²⁺-doped CIO were analyzed. 【Results】 Results show that Mn²⁺-doping just inhibited the formation of CIO, such as goethite and hematite, when R was less than 0.3, but promoted the formation of Mn-doped magnetite, some goethite and no visible hematite, when R was 0.5. In the CIO samples existed four types of hydroxyls, that is free hydroxyls, adsorbed hydrohydroxyls, surface associated hydroxyls and structural hydroxyls. With R rising from 0.1 to 0.3, relative content of the first two types decreased, while the latters increased. Adsorption peaks of the free hydroxyls and structural hydroxyls red shifted with Mn²⁺ doped, but those of adsored hydrohydroxyls decreased and those of surface associated hydroxyls behaved on the contrary when R increased from 0.1 to 0.3. When R was 0.5, structural hydroxyls almost disappeared, relative content of the surface associate hydroxyls varied between that of CIO and that of CIO-Mnx, and wavenumber of the adsorption band of surface associated hydroxyls was close to that of the CIO sample. Intensity and shape of the adsorption peaks of crystal structure Fe-O around 455 cm^-1 and 619 cm^-1 were related to the morphology of goethite, and those of the adsorption peaks around 478 cm^-1 and 560 cm^-1 were to the crystallinity of hematite. Intensity and wavenumber of the adsorption peak of hematite at 560 cm^-1 decreased when R increased from 0.1 to 0.3, and adsorption peaks at 478 cm^-1 disappeared with Mn²⁺ doped. When the R was 0.5, adsorption peaks almost disappeared at 543 cm^-1, widened and intensified at 474 cm^-1 and 593 cm^-1, and remained the same as that of CIO at 619 cm^-1. According to analysis, the vacant sites for cations in the structure of hematite in CIO-Mnx samples might get coupled with Mn²⁺ to form adsorption peaks around 567~589 cm^-1, of which intensity increased with rising R. In CIO-Mn_0.5 samples, Mn-doped magnetite formed with Mn²⁺ replacing Fe²⁺, thus forming a lattice vibrated Mn-O adsorption band around 593 cm^-1. 【Conclusion】 According to the results of the study for CIO and CIO-Mnx, a spot of Mn²⁺ inhibited the crystallization of CIO, which increased the varieties of hydroxy on the surface of CIO-Mn_x and changed the composition of hydroxyl in the samples. That the Mn²⁺ replaced the Fe³⁺adsorbed at vacancies in CIO caused a strong absorption around 567~589 cm^-1 in IR spectra. But the mass of Mn²⁺doping could change the CIO from goethite and hematite to Mn-doped magnetite, which appeared the different FT-IR characterizations of hydroxyl and crystalline structure.