Because　nodes　in　WSNs　have　limited　resources　and　usually　work　in　a　severe　dynamic　environment　without　human　participation,　existing　task　allocation　algorithms　in　WSNs　cannot　provide　fault-tolerant　mechanism.　Therefore,　a　new　task　allocation　algorithm　which　adopts　PSO　algorithm　and　multi-agent　technology　is　proposed　by　us.　The　algorithm　employs　primary/backup　copy　(PB)　technology　with　backup　copy　overlapping.　The　simulation　experiment　shows　the　proposed　algorithm　can　effectively　improve　task　guarantee　ratio　save　more　energy　and　prolong　the　lifetime　of　network.