The Ti551 alloy exhibits exceptional thermal stability, retaining over 80% of its room-temperature strength within 300-400 ^oC, outperforming most α-type Ti alloys. Consequently, it has emerged as the primary structural material for applications in extreme operating environments, such as deep-sea oil drilling pipes. Despite this advantage, the Ti551 alloy exhibits considerably complex thermal deformation behavior and microstructural evolution. Establishing the alloy's precise hot processing window is critical to broaden its engineering applications. Therefore, this study investigated the thermal deformation behavior of the Ti551 alloy using a Gleeble-3500 thermal simulation tester. Specifically, isothermal compression tests were conducted over the temperatures (T) of 800-1100 ^oC and a strain rate (\dot{\epsilon }) of 0.001-10 s^-1. Considering friction and temperature corrections, strain-compensated Arrhenius (SCA) and back-propagation artificial neural network (BPANN) models were selected to establish a constitutive model of the Ti551 alloy. The accuracies of both models were evaluated using the correlation coefficient, average absolute relative error, and relative error. The results demonstrate that the BPANN model outperformed the SCA model, yielding superior accuracy in predicting the flow stress. Thereafter, a hot processing map was constructed based on the dynamic materials model, and the corresponding microstructural evolution during thermal compression was systematically analyzed. Analysis of the hot processing map identified the optimal processing window with the following parameters: \dot{\epsilon } = 0.001-0.1 s^-1 and T = 900-1050 ^oC. Additionally, the instability zone was primarily concentrated in the high-T, high-\dot{\epsilon } region (T ≥ 1000 °C, \dot{\epsilon } ≥ 1 s^-1). These findings demonstrate that the deformation T and \dot{\epsilon } substantially influenced the flow stress. Specifically, the flow stress markedly increased with the decrease in T or increase in \dot{\epsilon }. During hot compression, the microstructural evolution of the Ti551 alloy exhibited a close relationship with T and \dot{\epsilon }, and the dynamic recrystallized grain size notably decreased as T decreased or \dot{\epsilon } increased. Furthermore, the dominant dynamic recrystallization (DRX) mechanisms during hot deformation comprised continuous and discontinuous DRX processes.