Accurately assigning formulas to thousands of peaks generated by ultrahigh resolution mass spectrometry in a single analysis poses a significant challenge, especially when dealing with diverse molecular compositions across complex mixtures. This difficulty is further compounded by the lack of an established universal mass calibration and formula assignment method. We have developed HRMS-Viewer, a Python-based software tool designed for processing ultrahigh resolution mass spectrometry data specific to petroleum and natural organic matter (NOM). The software employs an efficient, experience-driven approach for small molecule formula assignment, offering a streamlined yet intuitive workflow. Key features include advanced noise reduction, automatic or manual recalibration, real-time visualization of formula assignment results, and options for manual correction. During the workflow, HRMS-Viewer enables the visualization and manual control of critical steps including noise reduction, recalibration, peak identification, and data review.