Alloying　transition　metal　dichalcogenides　(TMDs)　with　different　compositions　is　demonstrated　as　an　effective　way　to　acquire　2D　semiconductors　with　widely　tunable　bandgaps.　Herein,　for　the　first　time,　the　large-area　synthesis　of　layered　HfS2(1-x)Se2x　alloys　with　fully　tunable　chemical　compositions　on　sapphire　by　chemical　vapor　deposition　is　reported,　greatly　expanding　and　enriching　the　family　of　2D　TMDs　semiconductors.　The　configuration　and　high　quality　of　their　crystal　structure　are　confirmed　by　various　characterization　techniques,　and　the　bandgap　of　these　alloys　can　be　continually　modulated　from　2.64　to　1.94　eV　with　composition　variations.　Furthermore,　prototype　HfS2(1-x)Se2x　photodetectors　with　different　Se　compositions　are　fabricated,　and　the　HfSe2　photodetector　manifests　the　best　performance　among　all　the　tested　HfS2(1-x)Se2x　devices.　Remarkably,　by　introducing　a　hexagonal　boron　nitride　layer,　the　performance　of　the　HfSe2　photodetector　is　greatly　improved,　exhibiting　a　high　on/off　ratio　exceeding　10(5),　an　ultrafast　response　time　of　about　190　mu　s,　and　a　high　detectivity　of　10(12)　Jones.　This　simple　and　controllable　approach　opens　up　a　new　way　to　produce　high-quality　2D　HfS2(1-x)Se2x　layers,　which　are　highly　qualified　candidates　for　the　next-generation　application　in　high-performance　optoelectronics.