The invention discloses an online monitoring system for the oil chromatography of a wind power dedicated box-type substation in the filed of power grids. The online monitoring system comprises an oil chromatography sampling system and a master control computer, wherein the oil chromatography sampling system comprises a sampling device, a degasifying device, a chromatographic column, a gas sensor and a data processor which are connected in sequence, wherein the sampling device is used for circularly sampling the oil of a transformer; the degasifying device is used for separating fault gases from the oil of the transformer; the chromatographic column is used for separating various characteristic gases in the fault gases according to components; the gas sensor responses the characteristic gases and converts the types and the concentrations of the characteristic gases into electric signals with different frequencies and different intensities; the data processor converts the electric signals into digital signals; the master control computer is connected with the data processor, reads the digital signals from the data processor periodically, calculates the content of each characteristic gas and the content of total hydrocarbons in the fault gases in the oil of the transformer, and determines a fault type and a fault position of the wind power dedicated box-type substation. The invention also discloses an online monitoring method for the oil chromatography of the wind power dedicated box-type substation in the filed of power grids.