The ventilation rate of a museum showcase is an important indicator for evaluating its sealing performance. In order to evaluate the ventilation rate of a showcase accurately, objectively and scientifically, some details of the detection and evaluation methods were comprehensively studied on the basis of GB/T 36110—2018. The results show that in order to achieve a better fitting effect, the carbon dioxide concentration outside a showcase should be fixed—a fixed concentration between 300×10-6 and 600×10-6 (volume fraction) had little influence on the fitting results. The average ventilation rate of the showcase at each moment was obtained by the numerical calculation method. When the detection time was short, values were unstable and fluctuated greatly; after stabilization, they were close to the fitted ventilation rates. The initial carbon dioxide concentration had a great influence on the fitting results of ventilation rate. The initial concentration of carbon dioxide gas in a showcase should be basically fixed between 20 000×10-6 and 25 000×10-6 (volume fraction) during detection, so that the testing results were comparable. The carbon dioxide mixture in the airtight showcase is relatively uniform, and no obvious dip occurred. The location of detection equipment has little influence on the fitting results of ventilation rate. If the fitted R² reaches 0.9, the results are considered valid, due to various errors in the detection process.