In the carpet tufting machine, feed milers are used to guide the yams positively. Mter deducing the general Euler formula of yarn tension under positive guide of rollers, a conclusion is got that the tension of yarns at the output side of each roller tends to decrease one by one. Afterwards, a computing formula of wrap angle of yarn passing through the feed roller is deduced by application of triangle geometry method. By analyzing the effect of each parameter on the yarn tension, values of the space between rollers and the roller diameter are proposed for design reference, and relations between the yarn tension and the two key parameters, the friction coefficient of roller surface and the speed of rollers, are illustrated in detail. Consequently, a policy is drawn out to control the yarn tension. The results of this paper provide not only theory foundation for empirical settings of the operational parameters of tufting machine, but also theory support for further research on yarn tension-control methods.