Abstract: This paper introduces the basic principle of PSTN video-phone and V.80 protocol.By analyzing the Recommendation V.80 which is in-band DCE control and synchronous data modes for asynchronous DTE,the author implemented an effective control model of synchronous trans-mission for the characteristics of low bit-rate multimedia communication on PSTN.It improved on the performance of H.324 videophone(VP324) greatly,Based the control mechanics,the video phone system can interact with other products based H.324 effectively.The increasing performances of the VP324 system come from the following aspects:① Adjusting the video codec strategies dynamically to adapt to current real communicating bandwith.It can not only use the bandwith sufficiently,but also prevent data overflow of multiplexing layer,and reduce the delay time of media signal;② Putting forward an Autobalancing Technology for audio-video processing which makes the picture more smooth and clear.It vacate more bandwith spaces for video stream by transferring the audiocodec policies which reduce the occupancy of audio streaming bandwith between 5.3k/6.3k and 2.4k;③ Reducing the effect on lip-synchronization to the lowest level due to bandwith waving dynamically,Through the V.80 control module cooperating with H.223 module and media codec modules.VP324 system already passed the professional accreditation of National Information Industrial Bureau,its all performance indexes achieve the national accredited requirements,and some of them exceed the other products based H.324,such as the highest frame rate is up to 25 fps under PSTN 28.8k bandwith.