Abstract: According to the need of practical use,a type of open microwave reactive cavity with plasmas,which is often used in the system of microwave-excited plasmas,is studied by use of the theory of open cavities.The effect of plasma densities and plasma collision frequencies on the field amplitude,field phase and reflection coefficient of the cavity is computed and analysed.It is clearly shown that the absorption of plasma to microwave is increased with the enhancement of the plasma collision frequency and that the standing wave pattern advantaging production of microwave plasmas can be effectively formed in that type of open structure.