According to determining the mercury in spinach powder, the possible uncertainties were analyzed and evaluated. The mathematical model of uncertainties was established to quantify and synthesize the components of uncertainty. The extended uncertainty of mercury in spinach was obtained by automatic mercury analyzer. The results showed that the mainly uncertainty sources of final results were successively determination process of mercury quality in samples, the measurement deviations, the weigh deviations and measured repeatability. The contribution to the uncertainty of the determination process were successively standard curve fitting, preparation of stock solution and standard series working solution. Operating strictly follow the rules and calibrating appliance regularly can reduce uncertainty. The mercury content in spinach powder is expressed as (26.061.67) µg/kg.