A start-stop control method of a bypass-free wet desulphurization system belongs to the technical field of wet desulfurization desulfurization system. An absorption tower slurry and original ash are preloaded for a startup process; a startup sequence is as below: successively putting an induced draft fan, a blower, an electrostatic precipitator, a demister flushing water pump and an oxidation air blower into operation; then igniting a boiler; and then starting an absorption tower slurry circulating pump, a desulfurization auxiliary system and SO2 removal rate adjustment. A stop process is as below: after flameout of the boiler, first stopping an absorption tower limestone slurry feed system, the blower and the induced draft fan; then releasing an absorption tower entrance accident spray system interlocking; and then stopping the absorption tower slurry circulating pump, a demister flushing system and the electrostatic precipitator. The startup and stop alternative control method for the desulfurization system and the boiler can reduce the dust and dirt entering into an absorption tower slurry pool, avoid influence of high temperature flue gas on the absorption tower, reduce discharge of the absorption tower slurry or waste amount to save the follow-up treatment costs, while effectively guaranteeing normal start and stop of the bypass-free wet desulfurization system.