The invention relates to an airbag device for detecting expansion joint performance between a steam turbine and a condenser. The airbag device comprises an expansion joint connecting a low pressure cylinder shell of the steam turbine and a condenser shell. The airbag device also comprises an airbag structure arranged on the periphery of the expansion joint. A cavity chamber is formed among the airbag structure, the expansion joint, the low pressure cylinder shell and the condenser shell. The cavity chamber is provided with at least two openings capable of being opened and closed. Isolation of large amount of valves can be avoided, and injection of huge amount of desalted water into a steam chamber of the condenser is also avoided so that lots of defects in water injection and vacuum leakage detection modes are avoided, and main equipment is ensured to be in good conditions. After replacement of the expansion joint, leakage detection via piping helium can be carried out, and temporarily constructed objects like internal scaffolds do not need to be removed. The helium leakage detection mode is high in accuracy and easy to locate leakage points, and the found leakage points are immediately eliminated. Compared with water injection and vacuum leakage detection modes, construction period is greatly shortened.