In this paper, we propose a fast video mixing method for reducing the computational complexity in the MCU (Multipoint Control Unit) used in the video conferencing. The conventional mixing method is based on the pixel-domain transcoder, of which computational complexity is linearly increased as the number of participants is increased. Basically the method requires many decoders and one huge encoder to mix the multiple bitstreams. To reduce the computational complexity, we propose a hybrid mixing method based on the syntax-based bitstream modification and pixel-domain transcoder. The proposed method reduces the computational complexity about 45% at the improved quality, compared with the conventional mixing method based on the pixel-domain transcoder.