The invention provides a gas chromatography online analyzing method and a device for a product of an ethylene reaction by biological ethanol dehydration. The method is to divide the product of the ethylene reaction by the biological ethanol dehydration into two paths, one path enters a product collection device; the other path enters a chromatographic column through carrier gas, wherein when the temperature is in a range of between 80 and 150 DEG C, ethylene, water, acetaldehyde, ethanol, ether and acetone are separated by the chromatographic column; and the separated components are introduced into a thermal conductivity cell detector, the contents of the ethylene, the water, the acetaldehyde, the ethanol, the ether, the acetone and other side products are detected at the temperature of between 200 and 250 DEG C. The method overcomes the defects that, in the conventional analyzing technology, the product generated after the biological ethanol dehydration has complicated operation and large labor intensity, the reacting status cannot be observed in time and the like. The response values of the components to be detected can be close in the chromatographic condition, and relative mass percentages of the components are approximate to the mass percentages of the components by using the area normalization method, so the qualitative measurement on the ethylene, the water, the acetaldehyde, the ethanol, the ether and the acetone can be completed in one time, the method can be used for automatic online analysis of the ethylene reaction by the biological ethanol dehydration.