Abstract: When using the graph multiplication method to calculate the displacement of beams of variable cross-section, the graph segmentation is complicated, which often leads to errors. This paper puts forward a stiffness reduction method, a bar of variable cross-section with a certain length can be transformed into two bars of constant section with different lengths, through the relationship between the stiffness and the displacement, and with the superposition, the equivalent displacement can be obtained. This method can be widely applied to the displacement calculation of rods with variable section, to significantly reduce the number of graph multiplications, and the amount of calculation, improve the calculation efficiency and accuracy.