The horizontal continuous casting process, the initial step in TP2 copper tubular processing, directly determines the microstructure and properties of copper tubular. However, the process parameters of the continuous casting characterize time variation, multiple disturbances and strong coupling. As a consequence, their influence on a casting billet is difficult to be determined. Due to the above issues, the common factor and special factor analysis of the factor analysis model were used in this study, and the casting experiment and billet metallographic experiment were carried out to diagnose and analyze the reason of the microstructure inhomogeneity. The multiple process parameters were studied and classified using common factor analysis, the cast billets with abnormal microstructures were identified by GT2 statistics, and the most important factors affecting the microstructural homogeneity were found by special factor analysis. The calculated and experimental results show that the principal parameters influencing the inhomogeneity of solidified microstructure are the primary inlet water pressure and the primary outlet water temperature. According to the consequence of the above investigation, the inhomogeneity of the copper billet microstructure can be effectively improved when the process parameters are controlled and adjusted.