The invention discloses a low-tension viscosity-reducing oil displacement agent, which is used for common heavy oil reservoirs, has a good viscosity-reducing effect, has good compatibility with formation water, can form ultra-low interfacial tension with crude oil, and is prepared from the following components by weight: 30-50% of an alkyl alcohol polyoxyethylene ether sulfate salt, 15-25% of a nonionic surfactant, 15-25% of an anionic surfactant, 2-5% of an organic solvent, 0.1-3% of sodium chloride, and 5-25% of water. According to the present invention, during the use, the low-tension viscosity-reducing oil displacement agent and the polyacrylamide polymer form the composite oil displacement system, wherein the polyacrylamide polymer can provide effects of water phase viscosity increasing, water-oil mobility ratio reducing, conformance expanding and profile control expanding, and the low-tension viscosity-reducing oil displacement agent can provide effects of oil-water interface tension reducing, oil displacement efficiency improving and crude oil viscosity reducing so as to further improve the water-oil mobility ratio, such that the extraction rate of the common heavy oil reservoir can be substantially improved through the synergetic effect of the polyacrylamide polymer and the low-tension viscosity-reducing oil displacement agent.