The effect of hydrogen on catalytic coupling reaction of CO was tested at a temperature of 120 degreesC and residence time of 1.8 s. It was found that after introducing hydrogen to this system, ethanol was produced by H-2 reaction with ethyl nitrite. With the increase of hydrogen concentration, the conversion of CO and H-2 decreased and the selectivity of diethyl oxalate decreased too. The regeneration of the catalyst after removing hydrogen from the system was tested to prove reversible poisoning. The results show that the hydrogen concentration should be kept below 2 mol%.