In order to solve the problem that 2 N-span & 2N-support rotor system and a N-span & (N+1)-support one have too large vibrations when passing through their critical speeds,their test benches were established to simulate their operation process.The vibration characteristics of  two rotor systems connected with three kinds of coupling were studied.The results showed that if the 2N-support mode is replaced by the (N+1)-support one,the critical speeds of the rotor system can be reduced; the coupling among rotating shafts of the rotor system with (N+1)-support mode is stronger to easily excite vibrations of adjacent shafts; the vibrations of the 2N-support rotor system and the (N+1)-support one tend to be steady when connected with smaller stiffness couplings,so the coupling among rotating shafts tends to be weaker; for an example,the vibration amplitude of the measurement point 1 on the rotor system with 2N-support when connected with flexible couplings is reduced by 50% near the critical speeds of the system,compared with that when connected with rigid couplings.