The efficient timely and accurate orientation of focusing aim is the key of a focusing system. A new evaluation method called multi-object free-focusing algorithm was proposed. User may select any single object by changing windows. Synchronized detection method was used to solve the problem of no object in the window after scene changing. The comparative and analytical method for restraining noise was discussed. The quickly focusing mathematical model was established. The technique is better than all the other methods by experimental comparison. Self-adapt-alter-step whole-range-search strategy was studied. The system was set up successfully in the TMS320DM642 hardware and VC software platform. The abundant outdoor and indoor experimental results indicated the feasibility, the veracity and the validity of the system, and it has been applied to frontier defence products.