Additive manufacturing (AM) is a technology that makes materials into products through layer-by-layer stacking according to a digital model. Nevertheless, during the layer-by-layer stacking, the temperature change at different positions inside the model is complicated. To measure the temperature change at different positions inside the stacked structure printing model during the AM process, in this study, we embed the distributed fiber into the polylactic acid (PLA) material model by optical frequency domain reflectometry (OFDR). The temperature change at different positions inside the material model can be measured at any time during the printing process. Considering the effect of filling density on the temperature change in the model, the filling density is set as 20%, 40%, 60%, 80%, and 100%, respectively. The results show that the internal temperature variation in the model with the same density at different locations tends to be consistent during the printing process, and the temperature change range is 20--40 ℃. According to the temperature variation curves in the model at different filling densities, the printing process is divided into five typical stages, including fiber embedding, thermometer hole encapsulation, model filling encapsulation, model capping, and model temperature regression. After analyzing the temperature changes at the peak point of model filling encapsulation and at the completion point of model capping, we find that the maximum temperature at the AM core with a 100% filling density is 15 ℃ higher than that with a 20% filling density, and with the increase in the filling density of the model, the printing material further hinders temperature dissipation.