The present invention provides a long-chain alkene double bond rapid on-line determination system and method, the long-chain alkene double bond rapid on-line determination system includes an ozone generator, a sample gas generating device, an ozone decomposition reaction apparatus, a gas supply means and a reaction outflow gas detection means. According to the long-chain alkene double bond rapid on-line determination method, the long-chain alkene double bond rapid on-line determination system is used for dividing high purity oxygen into two paths, one path enters the ozone generator in a certain flow rate to obtain a certain concentration of ozone, and meanwhile the other path passes through the sample gas generating device in a certain flow rate to obtain a certain concentration of an olefin sample gas. Finally, two paths of the gas are merged to enter the ozone decomposition reaction apparatus, part of a reaction outflow gas enters the reaction outflow gas detection means for analysis, and remaining reaction outflow gas is discharged as exhaust. Different concentrations of ozone can be obtained by changing the flow rate of the high purity oxygen and the current of the ozone generator, ozone decomposition reaction products enter directly the reaction outflow gas detection means without off-line concentration, derivation and other steps, and the method is simple in operation, fast in analysis speed and high in measurement precision.