Oscillating combustion is one of classic phenomenon in SHS. But the cause of its formation in a set of complex processes is unclear yet. With a two-step chemical reaction assumption and effects of other thermal dynamic factors, an auto- oscillating combustion hes been gained in a solid SHS process on the macro- humogenous and micro- heterogonous model. Numerical solution shows that the change of chemical reaction is the main cause of the oscillating combustion.