The invention discloses waterborne carbon nano-tube slurry and a preparation method thereof. The preparation method includes the steps: a) reserving deionized water, enabling the reserved deionized water to account for 10-20% of the total weight of the deionized water, mixing and dispersing the rest deionized water, a dispersing agent A, a dispersing agent B, a wetting agent and a moisturizing agent in a dispersion machine to obtain premixed solution; b) adding carbon nano-tubes into the premixed solution acquired in the step a) to disperse mixture by the dispersion machine for not lower than30 minutes at the speed of higher than 200 revolutions/minute, and adding the deionized water reserved in the step a) in the dispersing process to obtain pre-dispersion carbon nano-tube slurry; c) grinding and dispersing the pre-dispersion carbon nano-tube slurry acquired in the step b) in a grinder 3-10 times, and ultrasonically dispersing mixture for 0.5-2 hours to obtain the waterborne carbon nano-tube slurry.