The invention relates to a technological process for producing ethanol through the steam explosion and fermentation of the trunks and branches of poplar trees. The technological process comprises the following steps: (1) performing raw material steam explosion pretreatment for 45-55 seconds by taking the waste trunks and branches of poplar trees as raw materials under the conditions that the pressure of steam explosion is 3.1-3.3MPa and the temperature is 210-220 DEG C; (2) preparing the mixed enzyme solution of cellulase, xylanase and pectinase; (3) performing hydrolysis treatment for 24-48 hours; (4) concentrating hydrolysate to ensure that the sugar concentration is 70g/L; (5) preparing fermentation medium; and (6) mixing Pachysolen tannophilus and Saccharomyces cerevisiae to ferment and produce ethanol to obtain fermentation liquid of which ethanol concentration is 3-6%, wherein the sugar and alcohol conversion rate of the fermentation liquid is up to 35-39%. By adopting the technological process, the over-exploitation of petrochemical resources can be reduced, the rising earth greenhouse effect can be alleviated and the social benefit is great; the cheap trunks and branches of poplar trees are pretreated and fermented to ferment and produce ethanol which can replace non-renewable resources such as petroleum and gas, the economic benefit is remarkable, and the market prospect is wide.