The work aims to study the mechanism of color difference in dyeing process of dark pulp molded product and the effect of co-refining of dyes and fibers on dyeing performance and strength, and propose the technology to improve the dyeing performance and strength of dark pulp molded products. Under the same number of revolutions, the effects of adding methods of dyes on fiber morphology, and strength, optical performance and dye uptake rate of dyed products were investigated. The dyes and fibers were co-refined to discuss the effects of different refining revolutions on the drainage properties of pulp and the dyeing performance and strength of dyed products. The experiment proved that co-refining of the dyes and fibers was conducive to improving the tensile strength and bursting strength of the dyed products, significantly improving the dyeing effects and effectively reducing the color difference of both sides of the dyed products. Compared with the traditional adding methods, the color difference of dyed products reduced from 2.53 to 0.20, and the dye uptake rate increased from 72.02% to 77.93%. Microscope image analysis showed that the fiber picks were more remarkable in the co-refined slurry. In addition, with the increase of refining revolutions, the optical performance and strength of dyed products were improved. Co-refining of dyes and fibers can enhance the beating effect and improve the dyeing performance and strength of dark pulp molded product.