Abstract: A gas loop that is being constructed in the Chinese Experimental Fast Reactor (CEFR) is simulated in a full scale. A sampling system and a feasible method are developed to realize a discontinuous online analysis for carbon monoxide and methane in argon cover gas system. By using a thermal conductivity detector in stead of a conventional hydrogen flame ionization detector, the exhaust gas from the outlet is directly discharged to a fume hood, thus eliminating the risk of contamination of the analytical laboratory by radioactivity that would be released in the latter case. The experimental results show that the adopted analytical procedure can meet the requirement of CEFR for routine analysis of CO and CH₄:its precision is better than 3%, its accuracy is better than ±5% as evidenced by analyzing a set of CO and CH₄ references, and its lower limit of measurement is 5×10^-6.