Abstract: This study evaluates the uncertainty in the determination of gross alpha and gross beta radioactivity in drinking water using the thick source method. Following the GB/T 5750.13-2023 "Standard Examination Methods for Drinking Water Part 13: Radioactivity Index," gross alpha and gross beta radioactivity in drinking water were measured. The measurement repeatability was considered as type A uncertainty, while factors such as count rate, residue mass, water sample volume, instrument detection efficiency, and recovery rate were considered as type B uncertainties. The measured concentrations of gross alpha and gross beta radioactivity were 0.66 Bq/L and 0.48 Bq/L, respectively, with relative combined standard uncertainties of 6.0% and 5.7%, and expanded uncertainties of 0.08 Bq/L and 0.06 Bq/L. The uncertainty introduced by the instrument's count rate (n) was the largest source of uncertainty and had the greatest influence. Measurement repeatability, the counting efficiency (ε) of the standard powder source, and the recovery rate of radioactivity (F) also had a significant impact. The total mass of the water sample residue (W), the mass of the residue used to prepare the sample source (m), and the volume (V) of the water sample had less influence.