The invention discloses a one-step preparation method of POY / POY combined yarns and device. The method includes the steps: spinning is carried out with a first spinning assembly using first spinning liquid; spinning is carried out with a second spinning assembly using second spinning liquid; yarns spun through the first spinning assembly and the second spinning assembly pass through lower yarn guide plates of yarn paths respectively, then are combined on an upper yarn guide plate, and then sequentially pass a wire stitching machine and a winding roller to obtain the POY / POY combined yarns. Improvement is made on the basis of an existing production device preparing combined yarns of pre-oriented yarns and fully drawn yarns in a one-step method, the improvement is easy and convenient, compared with the conventional combined yarns of the pre-oriented yarns and the fully drawn yarns, the POY / POY combined yarns prepared in the one-step method are soft in handfeel, materials are various, spinning is flexible, production efficiency is high, the technological process is short, energy consumption and material consumption are low, fixed length and fixed weight are achieved, product quality stability is good, and the POY / POY combined yarns prepared in the one-step method can be directly used for weaving, the combined yarns with different sections, different fineness and different constituents can be prepared, and the combined yarns can be used for simulation fabric or functional products.