Brake block is an essential part of auto brake parts,and its inner air bubbles seriously affect driving safety.Traditional sound detection method has strong subjectivity,low efficiency,difficult to meet the accurate and high efficient detection requirements.The detection method based on eddy current lock-in thermography is applied to detect air bubble in less metal friction material brake blocks as it has the characteristics of the non-contact and large area inspection.In order to investigate the effect of structure and inner air bubble on phase,finite element method is adopted.Taking back side steel phase as a reference,the air bubbles were separated.The brake block was tested,and the same defect also can be seen after water cutting.Experiment results show that eddy current lock-in thermography detection method can effectively detect air bubbles in less metal friction material brake blocks.