Soft coal tar pitch (SCTP) with low QI content is the preferred raw material for preparing coal-based needle coke, the study on its structure changes in the stage of liquid-phase carbonization into coke (350~550 ℃) is helpful to prepare high-quality needle coke. In this paper, a detailed analysis on the C—H stretching vibration peaks in the range of 3 100～2 800 cm-1 and the aromatic C—H bending vibration peaks in the range of 900～700 cm-1 has been carried by the peak-fitting technique using the infrared spectra of the sample. And then based on the calibration factors of the corresponding C—H vibration peaks for the standard substances, the mass percentages of the different types of aromatic hydrogen (Hsolo, Hduo, Htrio and Hquarto) and aliphatic hydrogen (HCH3, HCH2 and HCH) of SCTP were quantitated at different carbonization temperatures (400, 500, 600 and 800 ℃). Furthermore, the contents of SP2 hybridized Carbon (SP2C) and SP3 hybridized Carbon (SP3C) as well as these structural parameters such as H/C atomic ratio, aromatic index (Iar), aromatic ortho-substitution index (Ios) and branched index (CH3/CH2) were calculated, and the changes of aromatic structure of SCTP during the coking process were also discussed. The results showed that the coal-based needle coke raw material SCTP is mainly composed of aromatic hydrocarbons with a low number of the ring and few side chains, its Iar is 0.77, and about 82% of its aromatic hydrogen is distributed in the structure containing three/four adjacent aromatic C—H, while its aliphatic hydrogens are mainly distributed in the CH2 of naphthenic structures. With the increase of carbonization temperature, the aliphatic hydrogen or SP3C of SCTP decreased almost linearly, losing about 50% at 400 ℃, which was mainly attributed to the loss of light components and the dehydrogenation of naphthenic structures. The green coke formed at 500 ℃ had Only 0.15 Wt.% aliphatic hydrogen and 0.88 Wt.% SP3C, and the presence of aliphatic hydrogen was not detected at 600 ℃. However, the aromatic hydrogen increased slightly from 3.89 Wt.% of the raw material to 4.5 Wt.% before 400 ℃ because of the conversion of the naphthenic structuresinto aromatic rings. As the temperature increases further, aromatic hydrogen decreases rapidly, reaching only 1.14 Wt.% when the temperature reaches 500 ℃, indicating that the aromatic hydrocarbon molecules undergo intense dehydrogenation condensation reaction during the mesophase formation stage at 400~500 ℃, which was also confirmed by the conversion of a large number of protonated SP2C into unprotonated SP2C. Aromatic hydrogen continued to decrease after 500 ℃, and their presence was not detected at 800 ℃. In addition, it was found that the out-of-planebending vibration of aromatic C—H is more sensitive to infrared light than its in-plane stretching vibration. The increase of Iarand the decrease of these parameters such as H/C atomic ratio, Ios, CH3/CH2 indicated that the aromatic molecules in the SCTP are gradually grown up by their condensation, and its aromaticity increased in the coking process. The fast quantification of various types of hydrogen by infrared spectrum can timely understand the structural changes of aromatic hydrocarbon molecules of the pitch in the coking process, which is helpful to the production of needle coke.