The heavy-duty moving average window (MAW) method, used for heavy-duty diesel vehicle (HDDV) real driving emission certification, has been long criticized for its unreasonable results. To quantitively analyze the problem, causes, and impacts of the MAW method, five China VI HDDVs were tested under real driving conditions. The specific method and MAW method with different boundaries are applied for data analysis. The results illustrate that cold start occupied 40.82 ± 11.22% of the total NOx emission within 5.77 ± 1.21% of the duration. Compared to the specific method, the MAW result gap is observed varying from −16.92% to 100.24% and didn’t show any pattern. Three reasons could explain biased MAW results: the 20% power threshold excludes the cold data; the 90th accumulative percentile window brings large uncertainty to the result and leaves the highest 10% window without supervision; the initial data gets low utilization. The MAW method could lead to ineffective NOx supervision and exhaust cheating. The future emission limits and emission inventories based on these results are also less reasonable. The above-discussed three reasons and the cold start data exclusion should be considered together to consummate the MAW method. These results could be used for future emission legislation and NOx control optimization. •China VI heavy-duty moving average window (MAW) method was evaluated quantitatively.•The cold-start occupied 40.82 ± 11.83% of the total NOx within 5.77 ± 1.34% of the duration.•The MAW method weakens the real driving tests due to its ineffective NOx supervision.•The power threshold and 90th percentile window bring large uncertainty to the result.•The MAW boundaries are closely coupled and they should be reconsidered together.