A new research perspective is proposed to optimize the topology of truss structure by force cone method, which involves force cone drawing rules and growth rules. Through the comparison with the mature variable density topology optimization method, the effectiveness of force cone method is verified. This kind of new method is simple and easy to understand (no need to master complex structural optimization design theory). Besides, it is time-saving in finite element calculations, and can obtain an optimized truss layout easily. By drawing the force cone, its application on a large radio telescope's back frame structure shows that, compared with the existing one in terms of structural stiffness, Root Mean Square (RMS) precision, and beam stress distribution, the optimized back frame using the force cone method has higher stiffness, better RMS, more uniform stress, and lighter weight.