The invention relates to a device and a method for measuring the water consumption in a limestone-gypsum wet desulphurization device. The method comprises the following steps of: measuring an average flow rate F of a process water tank in a water replenishing process by using an ultrasonic flowmeter; acquiring opening instrument time t1, t2 ellipsis tn of a water replenishing valve of the process water tank at a test working condition time period in a desulphurization device DCS (Distributed Control System); calculating total opening time t of the water replenishing valve of the process water tank, wherein a formula is as follows: t=t1+t2+ ellipsis +tn; carrying out an experiment for 7 days according to requirements in DL/T 998-2006 standards, wherein T=7*24h=168h; and calculating the water consumption Q, wherein a formula is as follows: Q=(F*t)/T. The device and the method disclosed by the invention have the beneficial effect of solving the problems that many water supply units are employed for a desulphurization device, and a part of water supply units do not have testing conditions to cause that the water consumption of the desulphurization device cannot be measured.