An on-line analyzer for volatile organic compounds in gas, comprising: - a sampling device; - a gas-supply device; - a refrigerating device connected to the sampling device and the gas-supply device respectively; - a gas flow control device connected to the sampling device and the gas-supply device respectively; - an analyzing device connected to the gas flow control device; and - a data acquisition and processing device connected to the analyzing device, - wherein the sampling device comprises: - a 6-way valve; and - a filter, an inlet tube for internal standards, an inlet tube for external standards, a first solenoid valve and a second solenoid valve which are connected to the 6-way valve respectively, wherein the first solenoid valve is connected to the second solenoid valve, wherein the refrigerating device comprises: - a 12-way valve; and - a first dewatering tube, a second dewatering tube, a first concentrating tube and a second concentrating tube connected to the 12-way valve respectively, wherein both of the first dewatering tube and the second dewatering tube are connected to the second solenoid valve, wherein the refrigerating device also comprises a trapping tube for CO 2, wherein the second dewatering tube is connected to the 12-way valve through the trapping tube for CO 2, wherein the first dewatering tube, the second dewatering tube, the first concentrating tube and the second concentrating tube are disposed in a cryotrap, wherein the analyzing device comprises: - an oven; and - a first capillary column and a second capillary column, disposed in the oven and connected to the 12-way valve respectively; - a first detector connected to the first capillary column; and - a second detector connected to the second capillary column, wherein the first detector is a mass-spectrography detector and the second detector is a hydrogen flame ionization detector; wherein the gas-supply device comprises: - a nitrogen-supply device connected to a pressure reducing valve; - a helium-supply device connected to the 12-way valve; - an air-supply device connected to the second detector; and - a hydrogen-supply device connected to the second detector; wherein the nitrogen-supply device is connected to the pressure reducing valve, the helium-supply device is connected to the 12-way valve, and both of the air-supply device and the hydrogen-supply device are connected to the second detector. - a sampling device; - a gas-supply device; - a refrigerating device connected to the sampling device and the gas-supply device respectively; - a gas flow control device connected to the sampling device and the gas-supply device respectively; - an analyzing device connected to the gas flow control device; and - a data acquisition and processing device connected to the analyzing device, - wherein the sampling device comprises: - a 6-way valve; and - a filter, an inlet tube for internal standards, an inlet tube for external standards, a first solenoid valve and a second solenoid valve which are connected to the 6-way valve respectively, - a 12-way valve; and - a first dewatering tube, a second dewatering tube, a first concentrating tube and a second concentrating tube connected to the 12-way valve respectively, - an oven; and - a first capillary column and a second capillary column, disposed in the oven and connected to the 12-way valve respectively; - a first detector connected to the first capillary column; and - a second detector connected to the second capillary column, - a nitrogen-supply device connected to a pressure reducing valve; - a helium-supply device connected to the 12-way valve; - an air-supply device connected to the second detector; and - a hydrogen-supply device connected to the second detector;