In　this　paper,　we　consider　a　wireless-powered　communication　network　(WPCN)　where　one　mobile　hybrid　access　point　(HAP)　coordinates　the　wireless　energy　transfer　to　sensor　nodes　and　receives　data　from　sensor　nodes,　which　are　powered　exclusively　by　the　harvested　wireless　energy.　As　the　harvest-then-transmit　protocol　is　employed　by　sensor　nodes,　a　major　challenge　lies　on　the　tradeoff　between　achievable　throughput　and　energy　harvesting　opportunity　of　sensor　nodes.　Confronting　this　challenge,　we　develop　an　energy　threshold　approach　by　jointly　considering　geographic　locations　and　energy　states　of　sensor　nodes,　where　wireless　energy　transfer　occurs　when　none　of　the　sensor　nodes　in　the　range　of　data　transmission　has　more　energy　than　the　threshold,　otherwise　data　transmission　from　one　randomly　chosen　qualified　sensor　node　to　the　HAP　occurs.　By　comparing　the　range　of　energy　harvesting　and　that　of　data　transmission,　we　divide　the　network　topology　into　two　cases　for　throughput　analysis,　and　formulate　the　energy　states　of　sensor　nodes　as　Markov　chain　processes　with　different　energy　state　spaces　in　the　two　cases.　Through　monotonicity　analysis　of　achievable　throughput　and　probability　distribution　of　energy　states,　we　prove　the　existence　of　the　optimal　energy　threshold　that　maximizes　the　achievable　throughput,　and　find　that　the　achievable　throughput　under　infinite　battery　size　could　be　viewed　as　the　upper　bound　of　that　under　the　limited　battery　size.　Finally,　simulation　results　validate　theoretical　results　of　the　optimal　energy　threshold,　and　show　the　impacts　of　system　parameters　on　the　achievable　throughput.