The　management　and　control　on　NOx　with　growing　concerns　has　been　in　the　spotlight　of　the　field　of　atmospheric　pollution　treatment.　Flue　gas　denitrification　(De-NOx)　is　a　conventional　approach　to　reduce　the　emission　of　NOx.　The　reductant　ammonia　accounts　for　the　dominant　consumption　during　flue　gas　De-NOx,　and　it　can　be　prepared　by　anhydrous　ammonia,　aqueous　ammonia　and　urea.　In　terms　of　different　advantages　and　characteristics　respectively,　every　preparation　of　ammonia　approaches　has　been　applied　in　project　cases.　Taking　the　advantages　of　safety　and　cost-performance,　the　urea-based　approach　has　been　increasingly　important.　In　the　recent　decade,　the　urea-based　approach　has　seen　great　strides.　The　conventional　approaches　including　urea　pyrolysis　and　hydrolysis　were　focused　and　the　related　research　and　application　were　reviewed　respectively;　the　research　prospects　on　the　preparation　of　ammonia　by　urea　were　outlined.