Abstract: The present disclosure describes an early warning method for assessing the fatigue damage state of a tie rod of a diameter expanding machine, based on the oil pressure of a power cylinder. (1) An oil pressure sensor is installed on the power cylinder, and collects oil pressure data in real time. (2) Required parameters are input into an early warning system. (3) A health state indicator function model of the tie rod is constructed. (4) Calculating the constant term in a linear equation allows for the construction of a tie rod S-N fatigue curve equation at a 95% confidence level. (5) A cumulative damage factor and a current health indicator of the tie rod are calculated. (6) If the early warning system determines that the current health indicator of the tie rod is 60.5 or lower, an alarm is triggered.