The burning law of the grain affects the interior ballistic calculation of the solid rocket motor. The UG NX and Matlab software are used to build the model and write the program for the flat-bottomed conic-column grain. The function diagram for the derivative of combustion area to the flesh-thick and design variables is obtained. The results show that when the relative radius of circular groove belongs to (0, 0.1), the relative radius of the hole in the cylindrical section belongs to (0.09, 0.3) and the relative radius of the circular center of the circular groove belongs to (R₂, 0.44), burning law is progressive at first and then regressive. When the aspect ratio is 2.5, the burning law tends to be constant. The example shows that the combustion area is calculated accurately and the burning law is the same as the actual situation.