Low delay live is a challenging technical difficulty in video live broadcast. In this study, delay optimization is carried out from three perspectives: network push stream, first frame delay, and video coding delay. The details are as follows. Firstly, according to the network condition, the code rate of live pushing stream is adjusted adaptively. Secondly, the key frame caching algorithm is implemented to reduce the opening time of the first frame to 2~4 seconds. Then, a fast selection algorithm is improved for intra-frame prediction mode, the coding complexity is reduced by cutting down the number of candidate modes and the encoding time can be reduced by up to about 22% under the premise that video quality is basically unchanged. Finally, based on the above three optimization strategies, the screen live system is designed, implemented, and applied to classroom teaching to meet the low delay scenes of classroom screen sharing.