Abstract: The eonversion coefficient of a TWT can be reduced, when it works at under-voltage condition. Therefore if a section of helix which works at under-voltage condition is added to a TWT, the improvement of AM-PM eonversion coefficient can be expected,then a doubl taper helix technique is developed.This paper describes the double taper helix technique and presents a small signal design method. The actual TWT which uses the double taper helix proves that it not only raises the electron efficiency, but also improves the AM-PM conversion coefficient. Further experiment shows that this technique has other advantages, such as operation at two or three power levels and good depressed collector characteristics.