The invention relates to a desulfurization device with a tower interior flue gas condensing function and a water balance control method. The desulfurization device comprises a desulfurization tower, acooler, an entrance flue, an exit flue, a first slurry spraying pipe, a second slurry spraying pipe, a first slurry spraying pump, a second slurry spraying pump, a washing water tank, a washing pumpand a washing spraying pipe, wherein a demister is arranged in the desulfurization tower. A slurry filtering device is also additionally arranged and comprises a primary cyclone, a secondary cyclone and a backwashing filter. Slurry is taken out of the desulfurization tower and is treated by the slurry filtering device, then, clear liquid is used for washing the demister, and remaining concentratedliquor returns to the desulfurization tower. Under the condition that washing water volume of the demister is maintained unchanged, the clear liquid resulting from filtering is used for making up difference value between desulfurization water replenishing volume and washing water volume which is larger than the desulfurization water replenishing volume, and thus, the desulfurization device establishes a new water balance.