Abstract: For laser welded I-core steel sandwich plates, in the design formulas of the stiffness the influence of joint geometric parameters are not considered commonly. Therefore the accuracy is lack in practice. The three-point bending tests were investigated for the sandwich plates with six kinds of joint geometric parameter by self-designed device together with the finite element simulations. The aim is to research the effect of joint geometric parameters on the bending stiffness and shear stiffness. The results showed that the increase of weld width can significantly improve the bending stiffness and shear stiffness. However, when the weld width is greater than 60% thickness of core plates, the increase of stiffness became slow. The influence of root gap to the stiffness can be ignored. Based on these results, the corrected and simplified stiffness formula was offered for sandwich plate stiffness design considering joint geometry parameters.