Objective To propose a design method of isothermal chamber for tidal volume testing equipment. Methods According to the State Equation of Ideal Gas, the gas volume was accurately calculated and injected into an airtight chamber, and then the pressure and temperature changes in the chamber were measured. To keep constant temperature, the fine copper wire was padded into the chamber to absorb the heat produced by the gas compressed during the injection. Results For the chamber of 1L, 300 g/L copper should be filled at least and with not more than 100 mL gas to keep the temperature inside the chamber isothermal. The experiment system was then connected with ventilator to compare the measured tidal volume and given tidal volume. The results showed that the experimental error was within 2%, which proved the accuracy of this testing equipment. Conclusions The establishment of the physical model of isothermal chambers can accurately detect the tidal volume of ventilator, which provides a theoretical basis and data support for factory inspection and supply more choices for ventilator maintenance in clinic.