Abstract: The Mean-Stream-Line Method suggested by Prof Wu has been developed in Many ways in China. The basic approach of this method is to solve the 2-D flow by correcting a certain 1-D flow of the so-called mean stream line. So, the arised errors will connect with deviation of the calculated flow from 1-D flow, I. e. the turning (curvature) and the converging (acceleration) of the flow, as well as the width of the calculated flow passage. According to this viewpoint, the error equations are obtained by the comparison between the analytical solutions of this approximate method and the rigorous solutions of the standard pure turning flow and pure converging flow. The calculated results of these equations are plotted to get a better understanding of this approach. It is seen, however, that this method is effective for most cases of modern subsonic and transonic turbine and compressor cascades.