Coaxial relativistic backward wave oscillator (CRBWO) is a new type of high power microwave (HPM) generator remarked by the conversion efficiency higher than 35%. It is found that in CRBWO the promotion of the beam quality, the synchronous harmonic distribution, and the collection of the spent electron on the inner conductor of the coaxial slow wave structure (SWS) accounts for the high efficiency which is confirmed by experiments. According to the special structure and distinguishing physical mechanism, a series of important improvements of the coaxial reflector and the structure with coaxial dual annular cathodes has been developed to enhance both the output power and the conversion efficiency up to 3.63 GW and 45%. And making full use of the both rippled conductors of CRBWO by installing an additional set of SWS into the inner conductor results in the generation of the beating wave with the average output power 5.88 GW and conversion efficiency 41.5%.