The invention relates to the technical field of water plugging for horizontal wells, in particular to a water plugging method for a thickened oil thermal recovery horizontal well. The method comprises the steps that firstly, nitrogen is injected to push stratum water to an oil-water boundary by means of the oil-water fluidity difference; secondly, a lignin water plugging agent is injected, polyacrylamide is adopted as carrier fluid, at the moment, the nitrogen injected in the first step can play a role in isolating stratum water from the lignin water plugging agent, and the lignin water plugging agent enters a stratum water onrush channel; thirdly, nitrogen is injected to eject the lignin water plugging agent to the bottom end of the stratum water onrush channel; fourthly, well shut-in and soak are carried out, and the lignin water plugging agent swells when encountering water to plug the stratum water onrush channel; fifthly, well opening and production are carried out. According to the water plugging method, oil production increment and water cut declination are achieved, the water cut declination effect is obvious, the application range is wide, the requirement for a completion method and water exit points is low, and the defects that an existing water plugging technology cannot meet the water plugging requirement of the thickened oil horizontal well and the phenomenon that oil production increment is not achieved during water plugging is likely to occur are overcome.