The invention relates to a working solution and a process for pretreating cotton/wool blended yarns. The working solution comprises H2O2, a scouring agent, an activating agent, bleach synergists and water; preferably, the mixture of anionic surfactants and a nonionic surfactant is selected as the scouring agent; tetraacetyl ethylene diamine or sodium nonanoyloxybenzene sulfonate is used as the activating agent; and perborate and percarbonate are used as bleach synergists. The invention also provides a process for pretreating cotton/wool blended yarns by using the working solution. By the working solution and the process, low temperature-bath pretreatment of the cotton/wool blended yarns can be realized, the process flow is shortened and the fiber damage is reduced, and energy can be saved.