Abstract: A temperature-jump accident occurred because of wrong operations for a propylene purifier in a chemical plant. To make sure if the purifier can be operated safely after the temperature jump, a series of testing and strength safety evaluation were implemented. Because the direct testing of mechanical properties of the purifier was not available, hardness measuring, metallographic examinations, ultrasonic inspection and hydro-test were performed, respectively. Simultaneously with hydro-test, stress measuring and metal magnetic memory nondestructive testing (MMMT) which can describe early defect were applied. The results showed that, due to temperature-jump, in the burnt area the grain size became larger, the colony of pearlite structures occurred, the hardness dropped by 30% and magnetic signal became abnormal as the stress increased. In order to monitor the magnetic change, the second MMMT was performed after the purifier had worked for a month. It is indicated that the reserve strength declined, the Tresca stress safety factor with normal working pressure of 2.5 MPa was about 1.92, several stress concentration existed in the burnt area and the level of stress concentration increased a month later.