The invention discloses a process for preparing synthetic ammonia by furnace gas generated when producing phosphoric acid in a blast furnace, comprising: bringing the furnace gas generated when producing the phosphoric acid in the blast furnace into a desulfurization procedure, then entering a catalytic conversion procedure to cause the reaction of CO and water vapor in the furnace gas so as to prepare H2, separating CO2 and N2 in the furnace gas, further removing residual CO in the furnace gas through a copper washing procedure, supplementing lacking N2 in the H2 according to a ratio of H2:N2=3:1, and entering a synthetic ammonia procedure after mixing. The invention uses redundant furnace gas generated when producing the phosphoric acid in the blast furnace to prepare the synthetic ammonia, coal and phosphorus resources are utilized to produce phosphoric acid by a blast furnace process, the redundant furnace gas is produced into synthetic ammonia, enterprises producing efficient ammonium phosphate compound fertilizer need not purchase synthetic ammonia, the capital investment can be saved by about 25 percent, and the invention has obvious advantages in production cost reduction,optimized operation and management.