According to the problem that the mass matter from rocket flame consumes the electron density in atmosphere and disturbs high-frequency electromagnetic wave propagation, based on the characteristic of high-frequency radar echo from rocket flame, this paper reports the physical mechanism of high-frequency electromagnetic wave attenuation and flame electromagnetic characteristics, and analyses the radar cross section characteristic of the flame and the procreant time of flame's high-frequency echo. Then the detection method of frequency energy agglomerated for flame is put forward. By simulation analysis and experimental observation, the relations among flame high-frequency echo, altitude, flame electromagnetic characteristic, electric wave frequency, angle of incidence are obtained. The detection method is reliable and can decrease false-alarm and missing-alarm.