Pyrolysis is a feasible way to convert lignocellulosic biomass into renewable fuels and chemicals. Comprehensive elucidation of chemical compositions of both intermediates and final products of biomass pyrolysis at the molecular level is essential to understanding the reaction mechanism, which may help to improve this sustainable process. Because biomass pyrolysis product is such a complicated mixture containing almost various kinds of organic oxygenated compounds, molecular characterization is challenging even with state-of-the-art analytical techniques. Online ultrahigh-resolution mass spectrometry (UHRMS) coupled with soft atmospheric pressure ionization (API) offers new opportunities in this context. Herein, we outline the instrumental concept of API-UHRMS dedicated to process analysis and its contributions to the molecular-level understanding of biomass pyrolysis. Recent progress in the online mass spectrometric characterization of the evolved gaseous products from pyrolysis of both complex raw biomass and three major constituents (cellulose, hemicellulose, and lignin) was summarized. In addition, the potential challenges and future perspectives in the context of analytical pyrolysis and process analysis are also discussed.