Collisions between detonations and shock waves of differentstrength have been numerically simulated by using NND scheme implementedwith the improved two-step chemical reaction model. The collision effects onthe detonation transition zone were investigated. In the case of detonationinteracting with weaker shock waves, the detonation transition is dominated byflow expansion which lowers the post-shock thermo-states and reduces thechemical reaction rate. The detonation transition can be divided into threestages, named as the collision effect zone, the detonation growth zone andthe steady detonation development zone respectively, which indicates somecharacteristics of detonation-decoupling and reigniting. When detonationinteracts with a stronger shock waves, the thermal properties of the explosivemixture are so high that the induced chemical reactions become more activeand the flow expansion is compensated. The transition appears to be a directtransformation from a weak detonation to a quasi-steady CJ detonation.