The invention discloses an online cleaning method for an off-gas pipe in a fluosilicic acid concentration system. The online cleaning method comprises the steps that firstly, diluted fluosilicic acid is led into an acid blending tank (1) and serves as an acid blending mother solution; afterwards, a circulating pump (2) is started to enable the diluted fluosilicic acid mother solution to pass through a cooler (3) and then to enter a Venturi (4) for spraying, so that total reflux operation is performed; after stabilization, an HF flow control valve (5) can be opened to lead in crude acid HF, flow is controlled to be 1-2 m<3>/h, and the acid blending temperature is not higher than 40 DEG C; leading in of the crude acid HF is stopped after the blended solution reaches 80%-90% the liquid level of the acid blending tank (1), and then acid blending is completed; when the off-gas pipe in the concentration system needs to be cleaned, a valve (9) between an outlet of the circulating pump (2) and the off-gas pipe is opened, a reflux valve (10) is closed, fluorhydric acid is fed into a gas-phase pipe (7) through an off-gas pipe nozzle (8), makes full contact with silicon dioxide and then flows into a concentration system (6) along a pipeline, and the cleaning effect can be completely achieved after spraying of 5 min-10 min. The online cleaning method guarantees continuous and stable operation of a device and fully utilizes raw material acid resources.