Flame retardant (FR) functionalization is widely recognized as one of the most difficult problems in the chemical processing of nylon 6 and 6.6 textiles. In the present work, potassium diphenyl sulfonate (KSS) originally used as an effective additive of FR polycarbonate was employed to improve the FR ability of nylon 6 fabric for the first time. A dipping adsorption process similar to acid dyeing was applied, and the adsorption and FR properties of KSS on nylon fabric were studied. The adsorption quantity of KSS depended greatly on the pH of its solution. The adsorption isotherm of KSS on nylon at pH 2 fitted a dual adsorption mechanism consisting of Langmuir and Nernst type models. The adsorption kinetic process of KSS followed the pseudo-second-order kinetic model. The limiting oxygen index and vertical burning tests demonstrated that a low dosage of KSS could impart good FR performance to nylon fabric. However, such FR nylon fabric suffered from poor washing resistance due to the good water solubility of KSS. The syntan originally designed for improving the wash color fastness of acid dyes on nylon could be employed to greatly enhance the washing durability of the FR nylon fabric.