Abstract: Quantitative analysis of tin content in infant supplementary food rice powder was conducted using external standard and standard addition methods. The study investigated the effects of different acids added during pre-digestion and complex matrices on the tin content results. An accurate method for determining tin content in infant supplementary food rice powder was established using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The study analyzed the sources of uncertainty in the determination process of infant supplementary food rice powder, evaluated each component of uncertainty, and calculated the method's expanded uncertainty. The analysis showed that pre-digestion with nitric acid followed by the addition of hydrochloric acid before microwave digestion, and the standard addition method for tin analysis resulted in a correlation coefficient (r²) above 0.9998. The method detection limit was 0.02 mg/kg, and the measurement deviation for certified reference material was +1.2%. When tin content was measured at 1.31 mg/kg, the expanded uncertainty was 0.04 mg/kg with a coverage factor of k=2. The standard addition method was suitable for determining tin content in infant supplementary food rice powder, proving to be more accurate and reliable.