The invention discloses a paper-based composite light-absorbing material for realizing efficient photothermal conversion and a preparation method thereof, carbon black particles in commercial carbon ink are deposited on the surface of filter paper by ultrasonic loading, the preparation method is simple, low-cost, green and environment-friendly, and the composite light-absorbing material with widespectral response range, high photothermal conversion efficiency and high hydrophilicity is obtained, the composite light-absorbing material is easy to be mass-produced and widely used, enriches the field of existing photothermal materials and fills the gap in the technical field. At the same time, based on the high-efficiency photothermal conversion property of the paper-based composite light-absorbing material, the composite light-absorbing material can be used for seawater desalination application, meets the society requirements of rapidly obtaining pure water, and provides a new way for solar thermal utilization.