Abstract: The shunt impedance of coupled cavity chain which is normally smaller thanthat of single cavity in the chain should be measured and calculated. A formula for direct calculating the shunt impedancc ZT~2 of the coupledcavity chain is presented according to analysis on the interaction between chargedparticles and electric field of cylindrical symmetry. The formula is a general form for calculating shunt impedance, which is alsovalid for single cavity. The method and results of experiment research on the formula are given, theresult obtained here are identical with that of Koichi Irie's formula.