Some diesel engines have been found to emit excessive amounts of NOx according to the recently established emission standards in Japan, primarily due to the poor performance of on-board catalysts. In this study, we investigated an evaluation method of the overall performance of an exhaust gas purification system in a heavy-duty vehicle that complies with Japan's latest emission regulations through activity tests of single and combined catalysts in a laboratory under simulated reaction gas conditions. The degradation rate of a simulated aged sample relative to a fresh sample was evaluated for a single selective catalytic reduction (SCR) catalyst and an SCR catalyst combined with a diesel oxidation catalyst and an ammonia slip catalyst. The results showed that the degradation of the SCR catalyst had a significant impact on the overall NOx purification performance of the system. The results and the evaluation method provide a useful reference for the improvement in the exhaust gas purification technology used in diesel vehicle engines.