Abstract: Many off-design condition phenomena of a turbojet engine are related with the variation of incidence angles at front and rear compressor stages.Based on the component matching regularity,the simple and clear analytical equations of the variation of incidence angles at front and rear compressor stages under engine operating conditions have been derived.With these analytical equations,a series of the off-design condition phenomena in a low or high design pressure ratio engines or in an,axial,centrifugal or combined axial-centrifugal engines,all well as in a two-spool engines are analyzed systematically,their physical natures are clarified,and various off-design condition problems which may occur can be predicted in advance.