Pressurized equipments maybe deform partially or wholly, and the mechanical properties of the construction material would be degraded due to a fire event. Fitness for service assessment can help to minimize reconstruction costs and allow safe resumption of unit operation as fast as possible. A propylene heat exchanger was exposed to overheating for about 3 hours due to a fire accident five or six meters far away. A fitness for service assessment was conducted according to API 579-1/ASME FFS-1. The material of the propylene heat exchanger is 09MnNiDR. The possible damage was examined by dimensional checks, hardness testing, in-situ field metallography, ultrasonic testing and magnetic particle testing. The heat exposure temperature of the propylene heat exchanger during the fire accident was estimated through the comparison between the results of in-situ field metallography examination and heat exposure simulation experiments. The heat exposure zones were identified based on the results of visual inspection and conjectural heat exposure temperature. The level 2 assessment was adopted to evaluate the heat exposure zones of V and VI. The approximate ultimate tensile strengths for the shell and the eastern head were converted from the hardness testing results. The caculated MAWP of the shell side is higher than the design pressure of the heat exchanger. The finite element method was adopted to evaluate the influence of the bulge in the upper part of the shell. The analytical results showed that the bulge had no significant effect on the operation of the heat exchanger before next inspection.