The invention discloses a method for removing ammonia nitrogen in biogas slurry. After being subjected to pH regulation, the biogas slurry enters a first liquid-state mixer; magnesium salt is added toform crystal nucleuses; then crystal nucleus growth is carried out in at least two parallel liquid-state mixers; and struvite precipitates are generated. According to the method, online ammonia nitrogen concentration regulation of a biogas project can be realized; obtained large-particle-diameter magnesium ammonium phosphate particles can be separated through precipitation; and the online addition mode reduces the usage amount of medicaments, thereby reducing the cost.