For solenoid DT4, the experiments of hot tensile and compress were carried out in the Gleeble 1500 to analyze the reason of cracks at the forging temperature 850-1150 ℃, and the phase transition point from α-Fe to γ-Fe of DT4 was measured. The experiment result shows that the phase transition temperature of DT4 is 900 ℃. When the temperature of hot tensile experiment is higher than the phase temperature such as 912，920，950 ℃, double neck-shrink phenomenon is observed and the temperature of neck-shrink point is almost the same as the phase temperature. When the temperature of compress experiment is 900 and 950 ℃ respectively, abnormal flow phenomenon happens at the ends of specimen. Within 850-1150 ℃, there is deformation concentration on the phase transition zone from α-Fe to γ-Fe and the compress stress-strain curve displays that the strength at the phase transition zone is lower than that of high temperature and low temperature nearby. Therefore the deforming concentration and crack occur easily. Thus, the forging crack can be avoided in the forging process beyond the temperature of phase transition from α-Fe to γ-Fe zone.