The effect of water on the formation of polychlorinated dibenzo-p-dioxins（PCDDs）and dibenzofurans（PCDFs）from precursor was investigated by the experiments with a fixed-bed reactor：the reactant mixture with different moisture of 123-TrCB and CuCl2 for metal-catalyzed formation，that of 123-TrCB and fly ash for surface-catalyzed formation. The experimental results show that the water has the following effects：first，suppressing the formation of PCDD/Fs as less its yield;second，inhibiting chlorination reaction as lower chlorination degree;third，having different effect on PCDDs and PCDFs as lower ratio of PCDFs/PCDDs.The explanations might be the competitive adsorption of water vapor with TrCB on active sites of the fly ash and the catalysis of copper was weakened chloride because of its oxidative conversion to copper oxide.