The invention discloses an online detection method for dye liquor components (sodium hydrosulfite, indigo and sodium hydroxide) of a bundle type dye machine on the basis of an Omron programmable logic controller (PLC). An online detection system comprises an indigo dye liquor online detection device, a vulcanizing dye liquor online detection device, a calibration device, the PLC and an industrialcontrol computer. The online detection method collects dye liquor samples in a dye tank of the dye machine in a real-time online mode, utilizes the industrial control computer and the PLC to serve ascores, detects concentration of corresponding components of the dye liquor in an online timing mode by means of a potentiometric titration method, and detects the pH value and the temperature of the dye liquor in an online real-time mode. The system can clean pipelines after completion of every test. The online detection method can calibrate the indigo dye liquor online detection device or the vulcanizing dye liquor online detection device according to requirements to ensure accuracy of detection results. The online detection method can ensure detection accuracy of the dye liquor components to improve dye quality of dyed fabrics and enable production cost and labor strength of skilled workers to be reduced.