The invention discloses a one-bath dyeing technology for polyester-cotton blended fabric by natural vegetable dye. The one-bath dyeing technology has the advantages that the polyester-cotton blended fabric is subjected to low-temperature treatment and then a one-bath treatment technology, and boiling-off, bleaching and dyeing are completed once, so that a technological process is shortened, wastewater discharge amount is decreased, energy consumption is reduced, and the environment-friendly requirements of China on the textile industry, involving energy conservation, consumption reduction, wastewater reduction and waste gas emission, are satisfied; organic solvents are avoided in a whole dyeing process, so that human bodies and the environment are hardly endangered; one-bath treatment liquid is reusable, so that the wastewater discharge amount can be decreased; dyeing promoters are added into the one-bath treatment liquid and then gelatin is added into a subsequent sodium silicate solution, so that the dye uptake is increased, color difference is reduced and fixation effects are enhanced; since the sodium silicate solution is used for fixation after one-bath treatment, excellent fixation effects are achieved, the dye uptake can be above 95%, and the fixation rate can be also above 90%.