The invention discloses a method for measuring leakage amount of a bridge crossing steam seal of a reheating type composite cylinder turbine. An accident steam discharging system discharging channel connected to the middle segment of the bridge crossing steam seal at the position of a high-intermediate pressure inner cylinder of the turbine is utilized, a temperature measuring point, a pressure measuring point and a flowmeter are additionally arranged, a steam discharging valve is opened to measure steam parameters, and the steam leakage amount of the bridge crossing steam seal is accurately obtained through the Martin formula, so that the authenticity of the performance examination test result is ensured, and the actual cylinder efficiency and heat consumption of an intermediate pressure cylinder are correctly reflected.