Abstract: Blocking situation in Eurasia is a charact eristic circulation of at mosphere, and its formation is a typical catastrophic phenomenon of circulation under the background With that the circulation meridionality grows strongly over Eurasia even to the Northern Hemisphere, and the circulation pattern frans formed from zonal through meridional, and then to blocking in turn.In this paper, the nonlinear theory and method are used to study this catastrophe and investigate the mechanism of blocking high. The main ideas are that:(1) The formation of blocking high links with the fluctuation and oscillation of circulation mechanism of the at mosphere system, so as to examine the evolvement of the system state, and the effect of the order of truncated spectrum and heating up distributing.(2) The formation of blocking high links with the at mosphere system's circulation changing from meridional to blocking pattern, so as to examine the evolvement of circulation, and the effect of the order of truncated spectrum and heating up distributing. The two aspects mentioned above are combined to comprehend the mechanism for formation of blocking high.The new research is explored:(1) Synoptic wave is add to the model (Lorenz,1963) in which baroclinic atmosphere ultralong wave and zonal flow interact, so a new model which contains ultralong wave, zonal flow and synoptic wave is set up, and it is closed to the actual atmosphere.(2) By comparing and analyzing the spectral form of 14 variables which are already made with the new one of 22 variables that are set up by us, we examined the effect of order of truncated spectrum to system.(3) Considering the actual distributing of two oceans and two continents in the Northern Hemisphere, we chang the zonal homogeneous thermal field to zonal inhomogeneous thermal field containing two heat sources and two cold sources, for examining the effect of heating up distribution changing to system.(4) Figures among phase trajectory, stream function and Poincar section map are used to analyze flow system and circulation patterns, to analyze their transition one another simultaneously, thereby we discussed the formation of blocking situation by an example.