The invention discloses a method and device for detecting a leakage rate of a solid oxide fuel cell system on line. The method comprises steps of: cutting off fuel gas supply of an anode cavity, cutting off an exhaust line of the anode cavity and cutting off high-pressure air supply of a cathode cavity in the operation process of a solid oxide fuel cell; obtaining an open-circuit voltage and temperature of the solid oxide fuel cell; and determining a leakage rate of the solid oxide fuel cell system according to the open-circuit voltage and the temperature of the solid oxide fuel cell. Based on the technical solutions disclosed by the invention, the leakage rate of the solid oxide fuel cell system can be detected on line.