The utility model provides a tractive device is swung to to bamboo of being cut down trees, includes enclasping mechanism and pull rope, and enclasping mechanism includes that the base that has two open arms, two staple bolts, the setting that articulates on two arms respectively are used for making the open spring of two staple bolts between staple bolt and arm, is provided with the guiding hole that supplies pull rope to pass on the base, and pull rope passes through the bipartition branch line and is connected with two staple bolts respectively, holds tightly by the bamboo of cutting down trees two staple bolts of power messenger that two staple bolts were applyed in opposite directions when stimulateeing pull rope accessible bipartition branch line, the building -climbing alarm device is simple in structure, and convenient for operation.