The invention provides a method for synthesizing polysorbate-80 for injection, comprising the following steps: (1) taking sorbierite as a raw material, partially dehydrating the sorbierite under the conditions of an acid catalyst and a vacuum state, and carrying out crystallization and purification for three times on the dehydrated sorbierite to obtain primary high-purity dehydrated sorbierite II; (2) further dehydrating the high-purity primary dehydrated sorbierite II and then carrying out reduced pressure distillation and purification to obtain secondary high-purity dehydrated sorbierite III; (3) mixing the high-purity primary dehydrated sorbierite II with the high-purity secondary dehydrated sorbierite III in proportion to obtain high-purity dehydrated sorbierite IV; carrying out addition polymerization on the high-purity dehydrated sorbierite IV under the condition of a basic catalyst and ethylene oxide to obtain dehydrated sorbierite polyoxyethylene ether V; and (4) reacting the dehydrated sorbierite polyoxyethylene ether V with high-purity oleic acid under the action of an esterification catalyst and then refining the reactant of the dehydrated sorbierite polyoxyethylene ether V and the high-purity oleic acid to obtain the polysorbate-80 for injection. The liquid chromatogram (LC) of the polysorbate-80 for injection has two principal product peaks without other impurity peaks; and compared with the conventional craft products, the polysorbate-80 for injection is higher in purity, more stable in the technology, is controllable, meets the injection requirements in quality and has more reliable safety in clinical application.