Chinese ocean color and temperature scanner (COCTS) is the main sensor on the Chinese ocean color series satellites, which is used for real-time monitoring of the ocean color and sea-surface temperature of the sea areas around China and part of the global ocean. The calculation of atmospheric diffuse transmittance is required in the ocean color atmosphere correction which affects the accuracy of the atmosphere correction and ocean color information retrieving. An exact computation method of atmospheric diffuse transmittance is developed based on the vector radiative transfer equation (VRTE) which is solved by adding-doubling method. Comparison with the diffuse transmittance derived from SeaWiFS exact diffuse transmittance look-up table, proves that the relative error of this adding-doubling method is less than 1.5%, and when the observing zenith angle is less than 60°, the relative error is even less than 0.5%. Therefore, the adding-doubling method can be used to generate the exact atmospheric diffuse transmittance look-up table for the Chinese ocean color and temperature scanner (COCTS). Finally, using the developed vector radiative transfer code with adding-doubling method, we generate exact atmospheric diffuse transmittance look-up tables for COCTS.