A new underwater image restoration method under natural light is proposed according to the ocean optics definition of background light. Optical parameters of water (attenuation coefficient and dissipation coefficient) used to calculate the background light are estimated based on reasonable hypothesis and deduction of optical theory formula. The relationship between scattering coefficient and wavelength is used to calculate the values of transmission functions of three channels (red, green and blue), and guided filtering is used to refine the transmission image. Finally, the underwater image is reconstructed with an inverse solving imaging model. Experimental results show that the proposed algorithm has some advantages in restoring the original color of the scene and removing the backscattering.