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 cooking process is helpful to prepare high-quality needle coke. In this paper, the CarbX software developed by the Smarsly team was used to fit the full spectrum X-ray diffraction (XRD) data of the samples toquantify the microcrystalline structure parameters of SCTP at different carbonization temperatures (400, 500, 600, 800, 1 000, 1 200 and 1 400 ℃), and then investigate the thermally induced structural changes of SCTP at the nanoscale. The results show that the average graphene layer size of microcrystalline stack L_a gradually increases from 10.3 Å for the pristine pitch to 47.9 Å at 1 400 ℃ with the rising of the carbonization temperature, but L_a increases slowly before 500 ℃. A significant increase of L_a is found only when the temperature exceeds 800 ℃, indicating that high temperatures above 800 ℃ are needed to recombine the atoms in the cross-linked graphene layers and lead to the growth of the microcrystals. However, the C—C bond length (l_cc) of the graphene carbon network is slightly affected by temperature and varies in the range of 1.41~1.42 Å. Because of mesophase transformation during the liquid-phase carbonization of SCTP into semi-coke, the average stack size L_c gradually increases before 500 ℃ and reaches the maximum at 500 ℃ (L_c=31.1 Å). Subsequently, due to further pyrolysis and polycondensation of semi-coke, L_c gradually decreases and reaches the lowest point (L_c=15.4 Å) at 1 000 ℃, and increases again after 1 000 ℃. Similar to L_c, the average number of graphene layers per stack N increases from 2.66 layers for the raw pitch to 9.05 layers at 500 ℃, then decreases to 4.55 layers at 1 000 ℃, and then begins to increase after 1 000 ℃. The samples are still in the pitch state before 500 ℃ the average graphene interlayer spacing a₃ is large, about 3.50 Å at this stage. When the pitch becomes semi-coke at ca. 500 ℃, a₃ rapidly decreases to 3.44 Å, continues to decrease, reaches the minimum at 1 000 ℃ (a₃=3.39 Å), and begins to increase again after 1 000 ℃, indicating that the coke has undergone a shrinkage and re-expansion process. By using CarbX software to fit the XRD data of the sample, the main size (L_a, L_c, N, a₃) of carbon microcrystals of the sample can be obtained, as well as the dispersion (k_a, k_c, σ₃, ε₃) of these parameters and the orientation (q), homogeneity (η) of per stack and disordered carbon content (c_un). It is helpful to deeply understand the sample's microstructure and to produce high-quality needle coke.