The　existing　research　on　three-phase　power　flow　in　ungrounded　distribution　networks　primarily　adopts　the　methods　of　presetting　zero-sequence　reference　voltages　and　neutral　point　reference　voltages.　This　makes　it　impossible　to　calculate　the　power　flow　in　ungrounded　network　with　non-zero　neutral　point　voltages　or　zerosequence　voltages.　Furthermore,　a　unified　zero-sequence　components　constraint　method　of　ungrounded　distribution　network　does　not　exist.　In　this　paper,　a　three-phase　power　flow　algorithm　is　proposed　to　calculate　threephase　power　flow　in　an　ungrounded　distribution　network.　First,　power　flow　equation　variables,　transformer　admittance　matrices　and　zero-sequence　components　are　used　to　analyze　the　characteristic　of　three-phase　power　flow　in　an　ungrounded　distribution　network.　Second,　a　three-phase　power　flow　algorithm　for　ungrounded　distribution　network　is　designed　by　constraining　the　zero-sequence　currents　of　the　ungrounded　winding　nodes　to　zero.　Third,　theoretical　analyses　and　mathematical　derivations　are　used　to　prove　the　applicability　of　the　two　methods,　one　of　which　constrains　zero-sequence　voltages　of　ungrounded　winding　nodes　to　zero　and　the　other　constrains　squares　of　zero-sequence　voltages　and　currents　to　zero.　Last,　a　five-nodes　network,　modified　IEEE123　network　and　practical　network　are　used　to　demonstrate　that　the　proposed　method　has　a　high　accuracy　and　good　applicability　in　calculating　the　three-phase　power　flow　of　ungrounded　distribution　networks.