Biomass, widespread and carbon-neutral energy, can provide electric energy and replace fossil fuel-derived production. Pyrolysis is the main way of converting biomass to different bioenergy products with the consumption of material and energy, which will cause environmental impacts. To confirm the actual environmental impact of biomass conversion, life cycle assessment (LCA) is used for analyzing the process. Due to choosing different LCA methods, the results for the same thing in different reports will show obvious fluctuation. This review is devoted to providing recommendations on how to handle methodological issues when analyzing LCA study, by which researchers can better realize the similarities and differences in biomass conversion system. In this review, multiple clarifications and methodological recommendations on the four steps of the LCA process (including goal and scope definition, life cycle inventory, life cycle impact assessment, and interpretation) are provided. Furthermore, the LCA results are discussed systematically, in which the global warming potential gets extra attention. Meanwhile, different biomass feedstocks are divided into agricultural residues, forest residues, and microalgae carefully. Finally, the current challenges and future framework of biomass conversion are expounded in detail from the perspective of LCA.