Abstract: The bifurcation dynamics of shallow arch which possesses initial deflection underperiodic excitation for the case of 1:2 internal resonance is studied in this paper. The whole parametric plane is divided into several different regions according to the types of motions; then the distribution of steady state motions of shallow arch on foe plane of physical parameters is obtained. Combining with numerical method, the dynamics ofthe system in different regions. especially in foe Hopf bifurcation region. is studied indetail. The rule of the mode interaction and the route to choos of the system is alsoanalysed at the end.