The high-temperature flow behavior and flow stress sensitivity of BT25y alloy were investigated. Results show that hot deformation is accompanied by the dynamic competition between work hardening and flow softening. The strain rate sensitivity exponent m tends to decrease with the strain rate after a first rise, and reaches the maximum at strain rate of 0.1 s−1. There is a large temperature range exhibiting m values above 0.2 at strain rates of 0.01–0.1 s−1. The temperature sensitivity exponent s shows an overall dropping trend with elevated temperature. The strain hardening exponent n first decreases and then increases with the strain at strain rate of 0.01 s−1. Large positive n values lie in areas with high strain rate, and small negative n values are located in areas with lower temperature and small strain rate. Secondary lamellar α appears near the phase transition temperature. The microstructure presents elongated characteristics at high strain rate.