The invention discloses an off-line washing system and method for a waste heat power generating boiler. A boiler drum and heat exchanging pipe boxes are connected to an inlet of a pollution discharge protopipe through pollution discharge pipes correspondingly, and a pollution discharge valve is arranged on each pollution discharge pipe; an outlet of the pollution discharge protopipe is connected with a pollution discharge expander through a connecting pipe, and an inlet of the pollution discharge expander is provided with a first switch valve; the heat exchanging pipe boxes are connected with flushing water pipes, an outlet of a boiler water feeding pump is connected with an inlet of the boiler drum through a water feeding pipe, and a second switch valve is arranged on the water feeding pipe; a pipeline of the connecting pipe is provided with a connecting branch pipe, the other end of the connecting branch pipe is connected to an inlet of the boiler water feeding pump, and a pipeline of the connecting branch pipe is provided with a third switch valve; and a pipeline of the water feeding pipe is provided with a water feeding branch pipe, one end of the water feeding branch pipe is located at the front end of an inlet of the second switch valve, the other end of the water feeding branch pipe is connected to an inlet of the flushing water pipe, and a pipeline of the water feeding branch pipe is provided with a fourth switch valve. The boiler water feeding pump is used as a flushing water pump, and thus investment is saved; and the boiler water feeding pump is high in lift, strong in flushing intensity and good in flushing effect.