To acquire the exact expression of the modified Beer-Lambert law for the transmission of Nd∶YAG laser in blood, the relationship between a geometry-dependent factor and blood depth must be clarified. In this study, we built a laser energy detecting system that can measure the transmitted Nd∶YAG laser energy using in vitro blood samples. Laser decay was measured in cylinder-shaped blood layer with a thickness of 1.0-2.5 mm, which is the typical blood layer thickness in clinical vascular diseases. Statistical methods and the R language were used to build linear, log, index, and power regression models of the geometry-dependent factor with blood thickness as an independent variable. The highest R² value among these four models is 0.9219, corresponding to the linear regression model. Based on the abovementioned results, the exact expression of the specially modified Beer-Lambert law has been built for Nd∶YAG laser transmission in blood.