Objective To evaluate the uncertainty of the aqua density average value of the sodium hydroxide volumetric sotution, which was prepared in the laboratory, and to establish the procedure and method to evaluate the uncertainty. Method Based on GB/T 601-2002, establish mathematical model, synthesize uncertainty. Results The primary factors that influenced the aqua density average value of sodium hydroxide titrating solution were found. The main origin of the uncertainty was induced and proposed and the procedure and method to evaluate the uncertainty of the density of standard volumetric solution by fiducial reagent were established. Conclusion The procedure and method of evaluation, which conform to the standard requirement, are useful to uncertainty evaluation in the routine similar experiments.