Focusing performance is an important parameter to improve peak power intensity of a laser facility. This paper provides a diagnostic method of focusing performance in order to realize a function of online diagnosing for focusing performance in petawatt laser. A high preciseness and low tolerance far-field diagnostics unit is designed in this method. Then measurement data is testified by a focal spot measurement unit in target room. At last the far-field diagnostics unit can be used to prove online diagnosing for focusing performance in petawatt laser. Experimental result shows that similarity between the far-field diagnostics unit and the focal spot measurement unit is about 1. So measurement data of far-field diagnostics unit can be used to diagnose peak power intensity with energy data and pulse width data when online diagnosing of peak power intensity in physics experiments is realized.