The horizontal continuous casting plays a key role in the production of inner grooved copper tubes. In order to improve the accuracy of the temperature field model of the copper tubes' horizontal continuous casting process, the model heat transfer coefficient was validated through temperature measurement experiment of graphite crystallizer. The finite element model of stress field evolution was established, based on considering the temperature and microstructure changes. It was found that tensile stress was generated in the outer layer of the casting billet and compressive stress was generated in the inner layer, when the casting billet entered the primary cooling zone. The paper investigated the mechanism of the liquid inlet number and shape on the microstructure and stress distribution after the casting billet was solidified. When the number of liquid inlets was 6, the ratio of the semimajor axis of ellipsoid to the short semiaxis was 3:2, and the backward tilt angle was 10 deg, the equivalent stress value of the casting billet was smaller, and the grains were dense and uniform. This paper promotes the research of horizontal continuous casting process and provides measurable reference for improving the quality of casting billet in the further.