This paper presents the interleaved quasi-resonant (QR) DC-DC flyback converter, which uses two parallel-connection DC-DC flyback converters. The interleaved technique is used to control the active switches of the two flyback converters. Also, the active switches are turned on at voltage-valley condition to reduce switching losses. Thus, this converter can achieve large output power, low output-current ripple, and high efficiency. The operating principle is discussed. Finally, a prototype circuit with input voltage 300-400 V, output voltage 24 V, and output current 8.33 A is implemented to verify the performance.