A simplified method is proposed for analyzing the overpressure history of an optional point on the walls of a closed cuboid due to its internal optional point-explosion. Firstly, the overpressure histories of all nodes on the walls of a cube with a side-length of 2 m are computed under a reference-charge explosion at each node of its inner space using the LS-DYNA software, and then are collected to form a reference database. Next, with the thought of the isoparametric finite element, an interpolating algorithm is established to calculate the overpressure history of an optional point on the walls induced by an explosion at any position inside this cubic space. Then, some ratio factors of peak values and durations of overpressure on the walls, reflecting changes in the charge weight and side-length of a cuboid, are derived and applied subsequently, together with their contributing coefficients, to make some modifications to the above algorithm, which achieves an approximate simulation to the overpressure histories on the walls under the optional charge weight and cuboid size. Finally, example results verify the rapidity and validity of this method,and provide feasible ranges of the charge weight and cuboid size according to the current computing condition.