A fogger in one of main devices used in forestry disease and insect control. A combustion chamber and exhaust tube(C.E.T.) is one of the main units of a fogger. The theoretical formula of calculating pulsating frequency of a C.E.T. is analyzed and deduced in the paper. It is proved that the test data of pulsating frequency of a C.E.T. will meet accurately the result calculated from the formula which provides the theoretical basis for C.E.T. designing.