Two-dimensional　(2D)　transition-metal　dichalcogenide　materials　(TMDs)　alloys　have　a　wide　range　of　applications　in　the　field　of　optoelectronics　due　to　their　capacity　to　achieve　wide　modulation　of　the　band　gap　with　fully　tunable　compositions.　However,　it　is　still　a　challenge　for　growing　alloys　with　uniform　components　and　large　lateral　size　due　to　the　random　distribution　of　the　crystal　nucleus　locations.　Here,　we　applied　a　simple　but　effective　promoter　assisted　liquid　phase　chemical　vapor　deposition　(CVD)　method,　in　which　the　quantity　ratio　of　promoter　to　metal　precursor　can　be　controlled　precisely,　leading　to　tiny　amounts　of　transition　metal　oxide　precursors　deposition　onto　the　substrates　in　a　highly　uniform　and　reproducible　manner,　which　can　effectively　control　the　uniform　distribution　of　element　components　and　nucleation　sites.　By　this　method,　a　series　of　monolayer　Nb1-xWxSe2　alloy　films　with　fully　tunable　compositions　and　centimeter　scale　have　been　successfully　synthesized　on　sapphire　substrates.　This　controllable　approach　opens　a　new　way　to　produce　large　area　and　uniform　2D　alloy　film,　which　has　the　potential　for　the　construction　of　optoelectronic　devices　with　tailored　spectral　responses.