This article analyzes a covered yarn, which consists of a core spandex yarn and an untwisted continuous viscose rayon yarn and discusses its mechanical properties by varying draft ratio, feed velocity, denier of core yarn and number of winds of covering yarn during the covering process. The helix angle θ can be determined by the above conditions.If the tension applied to the covered yarn during the covering process is removed, the covered yarn shrinks and reaches equilibrium state. The helix angle θb after shrinkage is determined by the initial helix angle θa in the covering process. The relation between θa and θb is induced.The formula on the breaking elongation of the covered yarn can also be deduced if the breaking elongation of spandex and viscose yarn is known.