The invention discloses an on-line detection device of a dye concentration in a printing and dyeing process of a connecting vat. The on-line detection device comprises a dye vat, an outlet valve, a cooler, a spectral analysis system and a return valve which are sequentially connected by pipelines and form a loop, wherein a cleaning pipeline is connected with the pipeline between an outlet of the outlet valve and an inlet of the cooler; an inlet of the cleaning pipeline is connected with a water tank; an angle valve is arranged at an outlet of the water tank; a three-way valve is arranged on the pipeline between an outlet of a constant flow pump and the return valve; and a connector at the bottom of the three-way valve is connected with a waste liquor discharge pipe. The detection device has the benefits that the detection device is applicable to printing and dyeing of the connecting vat, can effectively solve the problems that ingredients of dye liquor in each dye vat are unstable, aberration of the connecting vat is great and the color is unstable, can recycle detection dye, and can reduce the waste of the dye. The invention further discloses an on-line detection method of the dye concentration in the printing and dyeing process of the connecting vat. The method comprises the steps of 1, cleaning, 2, rinsing, and 3, data detection. The method is simple in step and easy to operate.