The invention discloses a carbon black nanometer solar energy absorbent applied to a solar water heater. The carbon black nanometer solar energy absorbent is prepared from medium water, a pH regulator, nanometer powder and a dispersant. The solar water heater comprises a transparent vacuum glass box for containing the prepared solar energy absorbent and heat conduction water pipes and a feeding opening. Slurry of the prepared solar energy absorbent is added to a glass box body, sunlight transmits the vacuum glass box and is irradiated on the slurry of the solar energy absorbent, nanometer particles dispersed in water absorb the sunlight efficiently by the adoption of the surface effect of the nanometer particles, and the water temperature is increased quickly. The carbon black nanometer solar energy absorbent mainly solves the problems that the wave absorbing gap positions among vacuum pipes are invalid; the selective absorbing materials are evaporated on the surfaces of inner pipes of the vacuum pipes, the solar energy absorption rate is low, the utilization rate is low, and the evaporation process is complex; and the cost is high, the heat dissipation speed is high, and the heat insulation effect is bad in the prior art and achieves the purposes that the efficient absorption and utilization rate of the solar energy reaches 85%-95%, the implementation process is simplified, and the cost is low.