Biomass is a renewable resource in the time scale of human life. Biomass resources are characterized by their enormous yield, wide distribution, renewability, and low-pollution nature. Recognized as the only practical and sustainable chemical carbon source for the future, as well as a crucial alternative to fossil resources, they provide essential support for energy utilization and product preparation. Bio-based products made from biomass resources are considered to be "carbon neutral", and its replacement of fossil products is an important means to tackle climate change. However, the entire life cycle process for bio-based products involves many carbon sources and carbon sinks, which challenges the assumption of "carbon neutrality" of bio-based products. Many bio-based product industries are energy-intensive and high-emission sectors, consuming substantial energy and generating significant carbon emissions. Additionally, the acquisition of biomass resources may involve land-use changes and associated carbon emissions. The end-of-life treatment methods for bio-based products exhibit substantial uncertainties in greenhouse gas emissions, particularly due to the complex carbon source-sink transformations under different disposal approaches. These factors contribute to ongoing debates over the assessment of the "carbon neutrality" of bio-based products, and call for a systematic framework to evaluate the carbon footprint of bio-based products. Based on the analysis of the characteristics of three types of biomass raw materials and bio-based products, this paper analyzes the carbon source and carbon sink processes of bio-based products in the life cycle, provides accounting methods for different carbon source and sink processes, and constructs a carbon footprint accounting framework of bio-based products. Taking printing paper made from wheat straw as an example, this paper analyzes the carbon source and sink in its whole life cycle and calculated its carbon footprint. Compared to only considering the production process (2633.38 kg CO2e), the carbon footprint of 1 ton of printing paper from wheat straw was 1221.50 kg CO2e. Carbon sinks formed by by-products such as agricultural fertilizer, waste paper landfill and paper product storage are important ways to reduce carbon footprint. Waste paper treatment methods have a significant impact on the carbon footprint of paper products. However, only changing waste paper treatment methods cannot achieve "carbon neutrality" of paper products, and the optimization of energy system and chemical inputs is still needed. In addition to the carbon footprint, the impact of bio-based products on water resources, land use change and biodiversity also needs to be comprehensively considered to realize the important role of bio-based products in addressing the triple planetary crisis of climate change, biodiversity loss, and pollution.