The invention relates to a device and method for preparing a desulfurizing agent, in particular to a device and a method for preparing an alkali 
slag liquid desulfurizing agent.  Including the alkali 
slag conveying pump, the alkali 
slag conveying pump is connected to the desanding device through the first pipeline, the top of the desandering device is connected to the thickener through the second pipeline, and the upper part of the thickener is connected to the collector through the first overflow pipeline.  The main pipeline is connected, the thickener is connected to the 
chlorine reduction tank through the fifth pipeline; the auxiliary agent tank is connected to the middle layer of the 
chlorine reduction tank, the washing water pipeline is connected to the lower layer of the 
chlorine reduction tank, and the bottom of the chlorine reduction tank is connected to the finished 
buffer tank through the sixth  Pipeline connection; the upper layer of the chlorine reduction tank is connected with the general collection pipeline through the second overflow pipeline.  The preparation method is: gravity sand 
settling, thick 
sedimentation, washing and dechlorination, and 
sedimentation after thickening. The liquid desulfurizer is discharged from the bottom of the chlorine desulfurization tank and stored in a 
buffer tank.  Compared with the existing technology: the application of 
high energy consumption equipment such as filter presses and centrifuges is avoided, the mechanical 
solid-liquid separation of alkali slag waste liquid is avoided, and the 
flue gas desulfurization effect is effectively improved.