The limit induced bifurcation (LIB) in AC/DC power system caused by converter transformer tap and extinction angle limit under the control mode of constant current in rectifier and constant voltage in inverter is studied in this paper. The reason why the singularity induced bifurcation does not occur while the hard limit encounters is analyzed. The transformer ratio should be larger than a critical value while LIB occurs and the formulas for critical value of LIB in defferent types are derived in this paper. The sensitivities of critical value to parameters are calcaulated, from which some bifurcation points are eliminated and the stable region is augmented. And, the effects of field voltage limit on critical value and stable region are discussed. Finally, a mechanism of limit induced bifurcation is proposed. The increase of reactive power consumed in converter transformer and the decrease of the reactive power compensation in converter AC bus and the transform of control mode result in the system destabilization due to the lack of reactive power.