The invention discloses a process for preparing ethyl trichosanate and a synergist of ethyl trichosanate as a herbicide, and belongs to the field of chemistry. The preparation process comprises the following steps: with recovered trichosanthes kirilowii maxim oil fatty acid as a raw material, filtering, acidifying, esterifying, purifying and the like. According to the synergist of ethyl trichosanate as the herbicide, the amount of ethyl trichosanate is 0.5%-1.5% of volume of herbicide spray liquid. The preparation process is mild in condition and environmental friendly; and the obtained product ethyl trichosanate is used as a novel efficient and low-temperature resistant herbicide synergist and has a good effect.