During the production of coal-based needle coke, the performance of the mixed oil fluctuates continuously in a coking cycle due to the delayed coking process. How to stabilize the performance of mixed oil is a key factor in ensure the uniformity of needle coke quality. The difference in the properties of mixed oil is mainly reflected in the change of viscosity. In order to quantitatively analysis of the change of mixed oil, the mixed oil with different continuous feeding time in the same production cycle has been detailed analyzed in this study. Briefly, the Fourier Transform Infrared Spectroscopy (FTIR) analyzer and the rotational viscometer have been used as the detection means, and six kinds of molecular structural parameters which calculated from the various range of FTIR spectrum (700～900, 1 550～1 650, 2 800～3 000 and 3 000～3 100 cm-1, respectively) were used as the significant factors to characterized the mixed oil. The correlation between the viscous fluid flow characteristics and the molecular structure of the mixed oil was discussed in detail. The results showed that FTIR spectrum analysis showed that the mixed oil was mainly composed of condensed aromatic rings with partial aliphatic side chains. The branching degree (I1) of aliphatic side chains decreased continuously, and the aromaticity (I2) in aromatic structure increased slightly in the mixed oil during the production of coal-based needle coke. However, the variation of aromatic ring condensation degree I3 and substitution of aromatic rings (I4, I5, I6) changed slightly, which indicated that the degree of condensation of blends changed little with the increase of coking time. The coexistence of multi-component complex aromatic substances in the mixed oil led to easy association among molecules, which made the initial apparent viscosity of the mixed oil was large. And the viscous fluid flow activation energy Eη increased with the prolongation of production time. In theory, the condensation aromatic rings and alkyl side chains have the greatest influence on viscous flow properties, but when I1, I2, I3 and Eη were analyzed, it was found that the goodness of fit of regression curve R2 can only reach 0.71. In fact, the branching degree of the mixed oil was low and the length of the branching chain was short. When the influence of I1 on the viscous flow activation energy was neglected, the goodness of fit R2 of the regression curve obtained by data processing of I2, I3 and Eη decreased. Considered all the molecular structure parameters and Eη for regression analysis, the goodness of fit of regression curve R2 can reach 0.98. The relationship between the viscous fluid flow characteristics and the molecular structure followed the following model: Eη=703.59-55.88I1-7.83I2+5.73I3-1 866.70I4-694.85I5-83.16I6. It can be seen that the viscous flow characteristics were the macroscopic manifestation of all the molecular structure characteristics in the complex system of mixed oil.