Abstract: Based on Standard JJF 1059.1—2012，and taking the content measurements of C，Si，Mn，P and S as examples，the source of the uncertainty in chemical element content measurement of linepipe-purpose steel with the photoelectric direct-reading spectrometer is analyzed，and accordingly related math model is established.Furthermore，the uncertainty of the content measurement result of each chemical element in the said steel is evaluated so as to determine and calculate the component products of each uncertainty as occurring in the measurement process.Relevant comparison result shows that Type B standard uncertainty is the main source of the uncertainty in spectroscopic analysis，while the uncertainty from reference sample value determination together with the uncertainty as induced by the spectrometer during the measurement process mostly affect the general uncertainty.