The invention relates to a method for detecting hydroquinone in compost on line, which comprises putting a biosensor in measured solution, utilizing an electrochemical analyzer to analyze current variation data which are adopted by the biosensor, getting response current change characteristic values, response current stabilization time and steady-state current values, outputting the parameter values in a neural network analysis apparatus, and getting the concentration value of hydroquinone after analyzing and calculating. The invention also relates to an on-line detection system which is used in the detecting method, which comprises a three-electrode device, an electrochemical analyzer and a neural network analysis apparatus, wherein the three-electrode device is connected with the electrochemical analyzer, and the electrochemical analyzer is connected with the neural network analysis apparatus, the three-electrode device utilizes a carbon paste electrode which decorates a laccase-Fe3O4 magnetic nano-particle cross-linking body to a working electrode. The one-line detection method and the on-line detection system have the advantages of convenience, high effective, sensitivity, strong anti-interference capability, low production cost and the like.