Polyester/cotton yarns are dyed at high temperature and pressure. Dyeing fastness usually does not occur as long as the dyestuff is chosen reasonably and the process conditions are reasonable. From the dyeing formula, it is not reasonable to use leveling plus 0 as leveling agent when dyeing with disperse dyestuff because the cloud point of flat and O is generally 60-70 °C, while the temperature of high temperature and high pressure dyeing is 130 °C. °C, although the dispersion. Dyes already contain diffusers (anions) and are used in the same bath as flattening plus O to increase the cloud point, but dye aggregation is unavoidable. As the dyeing temperature exceeds the cloud point of flattening plus O, the dispersion of the dyestuff is disrupted and gels form in the dye bath and are deposited on the fibers, producing color spots and stains, while the partially dispersed dyestuff is not actually dyed with polyester And adhere to cotton fibers. On the other hand, improper use of disperse dyestuff can also lead to reduced color fastness, especially for dark varieties. The more the amount of dyestuff used, the darker the color. When the amount of dyestuff exceeds the adsorption capacity of the dyed fiber, not only does the color not become darker, but also a lot of contamination is produced to affect the color fastness. The furniture tensioner disperse black S-3BL dye itself is a mixture of dyes. If the safety tensioner is not produced by a real dye factory, it may be of poorer quality. Whether the reduction of disperse dyes after dyeing is also key to color fastness. As for dyeing with reactive dyes, it is necessary to consider whether the dyeing process is reasonable, whether the reactive dyes are hydrolyzed, etc. Due to the use of the double bath method, it is possible to test the firmness of polyester dyed yarns. After dyeing cotton fiber with reactive dyes and then testing the fastness, problems can be found.