Objective To study the reliability of the formaldehyde diffusion tube as a standard gas generating source. Methods Three diffusion tubes with different diffusion ratios were selected and the diffusion ratios were determined using the reference method,viz. AHMT colorimetry, and the results were compared with those obtained by gravimetric analysis. The accuracy of standard gases generated by the formaldehyde diffusion tube was analyzed. Results The thermal decomposition efficiency of trioxane turning into monomolecular formaldehyde was more than 99%, and the relative standard deviation was 0.40%. The uncertainty of AHMT colorimetry was less than 5% with a confidence limit of 95%. Conclusion The formaldehyde diffusion tube was precise and reliable as a standard gas generating source.