The configuration for a Multiwave Cerenkov Generator is designed with the help of a 2 1／2 D fully electromagnetic relativistic Particle-in-Cell (PIC) simulation code. Good performance of the well configured device is showed by simulation under a wide range of operation conditions (U_d=300～500kV，I_b=0.45～10kA and τ_p =10～10²ns). The microwave is generated in X-band (～10Hz)with radiation efficiency in the order of 10%. The power level is approximately 300MW for its maximum.