To　broaden　the　application　of　ultra-high　performance　concrete　(UHPC)　in　engineering　fields　with　high-early-strength　requirements,　accelerators　were　used　to　accelerate　the　setting　and　hardening　of　UHPC.　This　study　investigated　how　alkaline　accelerator　(NA)　and　alkali-free　accelerator　(AS)　influence　the　strength,　hydration　and　microstructure　characteristics　of　UHPC.　To　achieve　the　proposed　objective,　the　terms　of　setting　time,　strength,　released　ionic　concentration,　isothermal　calorimetry,　X-ray　diffraction,　scanning　electron　microscope,　energy　dispersive　X-ray　and　mercury　intrusion　porosimetry　were　performed.　The　results　show　that　NA　and　AS　significantly　increase　the　setting　and　hardening　rate　of　UHPC,　and　their　optimal　dosages　are　2%　and　4%,　respectively.　2%　NA　increases　the　1　d　compressive,　splitting　tensile　and　flexural　strengths　of　UHPC　by　55.8%,　62.2%　and　56.5%,　respectively,　while　4%　AS　increases　by　10.8%,　21.6%　and　21.0%,　respectively.　However,　their　later　strengths　decrease　slightly.　The　first　exothermic　peak　values　of　NA-2%　and　AS-4%　are　about　3　times　higher　than　the　reference　paste,　and　their　main　hydration　peaks　appear　earlier　by　38.2　and　23.3　h,　respectively.　NA　and　AS　contribute　to　the　rapid　formation　of　hydration　products　and　refine　the　pore　structure　of　pastes.　©　2021　The　Authors