Abstract: To solve the problem that the traditional balance winding schemes require multiple trial and error adjustments, and the EMI design and electrical performance of transformers usually can't be satisfied altogether. This paper worked on the transformer with spiral windings. The causes of errors in the traditional balance winding design were analyzed and a balance winding technology with the shielding effectiveness was proposed, which could be achieved by adding a simple structure and easy implement improved balance winding to suppress the common-mode (CM) conducted noise of the flyback converter effectively. The mechanism of eliminating the CM displacement current on the transformer transmission path was quantitatively analyzed, and the design criteria of improved balance winding were proposed. Experimental results show that under the condition that the transformer windings are interleaved, the improved balance winding only needs to occupy one-layer winding space of the transformer, and it can achieve the lowest CM interference amplitude. The manufacturing process of transformers in the EMI optimization design is effectively simplified.