The invention provides a method for preparing a 
magnesium nitrate solution by using 
nitric acid tail gas, which comprises the following steps: respectively injecting 
nitric acid tail gas after being washed by a washing 
tower and water on 
magnesium oxide surface to form a solution; uniformly introducing 
compressed air and then stirring to generate the 
magnesium nitrate solution; filtering the 
magnesium nitrate solution and then introducing in the washing 
tower and repeatedly generating the 
magnesium nitrate solution to increase its concentration; deposing for 8-10 hours, and filtering to obtain the finished product 
magnesium nitrate solution. The invention also provides an apparatus. According to the invention, a non-pressure process and a pressurization method are used for producing dilute 
nitric acid, 
tail gas generated by dilute nitric acid contains a lot of 
nitrogen oxides such as NO and NO2, the 
nitrogen oxides are washed by the washing 
tower to remove the impurities, and then added in magnesium 
oxide with water and 
compressed air, so that 
nitrogen oxides, water, and alkalescence suspending liquid prepared by 
oxygen and magnesium 
oxide are reacted to directly generate the magnesium 
nitrate solution. According to the invention, reaction is mild, acid mist and 
nitrogen oxides are not generated, so that personal safety is ensured and environment can not be polluted, and magnesium nitrate concentration can be gradually increased to 40%.