Abstract: 8 kinds of mixed oil with different feeding time in the same coking cycle were the objects of the study. The optical structure, morphology structure and microcrystalline structure of needle coke formed by mixed oil with different feeding time in coking cycle were quantitatively analyzed by means of polarizing microscope, scanning electron microscope (SEM), XRD and Raman spectroscopy. The results show that the yield of coke after the thermal conversion of the mixed oil is higher than the theoretically calculated yield, indicating that the heavy oil participates in the thermal conversion reaction to form the coke with a streamlined optical structure. In the optical structure of calcined needle coke, MO-8" has the highest fiber content, followed by MO-16", and MO-32" has the lowest fiber content. SEM further proves that MO-8" has more lamellar number and more regular orientation. XRD analysis of needle coke confirms that the microcrystalline structure parameters (interlayer spacing d₀₀₂, lamellar content N and the number of carbon atoms in each layer n) are very close. However, there are obvious differences in the content of graphite microcrystalline (I_g), and among which MO-8" has the highest content, followed by MO-4", and sample MO-32" has the lowest content. Furthermore Raman spectral analysis certifies that the basic microstructure of needle coke is similar. The fundamental reason is that the refined coal-tar pitch in the mixed oil determines the basic microstructure of needle coke. Due to the continuous circulation of heavy oil in the system, the microstructure of needle coke are different. Hence, the coking cycle is not easy to exceed 32 h in coal-based needle coke production, or else it will seriously affect the microstructure of needle coke.