Scalable Video Coding (SVC) aiming to provide the ability to adapt to heterogeneous requirements. It offers great flexibility for bitstream adaptation in multi-point applications. However, transcoding between SVC and AVC is necessary due to the existence of legacy AVC-based systems. This paper proposes a fast SVC-to-AVC MGS (Medium-Grain quality Scalability) transcoder. A quantization-domain transcoding architecture is proposed for transcoding non-KEY pictures in MGS. KEY pictures are transcoded by drift-free architecture so that error propagation is constrained. Simulation results show that proposed transcoder achieves averagely 37 times speed-up compared with the re-encoding method with acceptable coding efficiency loss.