Abstract: With the rapid development since the Fifth Assessment Report (AR5), observational constraints have become an important part of the lines of evidence used in the Sixth Assessment Report (AR6) of Intergovernmental Panel on Climate Change (IPCC) Working Group I (WGI) to improve the constraints on projections of future climate change. For the first time in the history of the IPCC, the WGI AR6 report uses multiple lines of evidence, including observational constraints based on past simulated warming, multi-model projections, as well as the updated climate sensitivity, to constrain the assessed future changes in global surface temperature, which reduces the uncertainty of the multi-model estimation. This paper reviews and introduces several main observational constraint methods and their applications involved in the WGI AR6 report, including model weighting, attribution-based constraint (ASK methods), and emergent constraint. No matter in AR6 or in many projection studies for different variables over various regions, observational constraints have shown the potential to correct model bias and improve model projections. In comparison, there are very few studies focusing on observational constrained projection in China. It is urgent to strengthen researches on observational constraint methods and their application in the projections of regional climate change in China, to provide more abundant climate information with narrowed uncertainty for policy formulation and adaptation planning in response to climate change.