Objective To study the mechanical principle of pleural indentation (PI) caused by lung cancer. Methods By using the software maths-"Matlab" and Finite Element Analysis-"Ansys", a biomechanics model of the lung was set to analyze the variation of PI in displacement, stress and strain under the pressure of thorax on the basis of clinical CT images. Results (1) The indentation part influenced by the cancer showed little deformation withthe minimum of 0.424 mm, while the indentation part far from the cancer has relatively much more deformation with the maximum of 60.248 mm. (2) During the inhalation, the bigger the Young’s module is, the more the degree of the indentation is; during the exhalation, the bigger the Young’s module is, the less the degree of the indentation is. (3) With the clinical CT picture we explain why the degree of the indentation during the inhalation is more than that during the exhalation. Because during the inhalation, the lung expands and the direction of deformation is opposite to that of indentation, which results in the growth of the depth of the indentation, while during the exhalation, the lung shrinks and the direction of deformation is the same as that of indentation, which brings on the decrease of depth of the indentation. Conclusion The principle for the formation of PI was explained based on the mechanics, which also accords with the clinical circumstances.