The paper suggests a space rigid frame model which is equivalent to the space flexibility of pile-group foundation. Firstly, calculating the flexibility of pile-group foundation with “m” method, then the formulas to calculate geometrical parameters such as height of the rigid frame, length and width of the column section, as well as distance between the columns are deduced. So the equivalent rigid frame is acquired exclusively by these parameters. Finally ,discussing the accurate simulation of vertical flexibility and tensional flexibility. Which are achieved by additional springs.