A new method to measure the thermal lens of the solid state laser is presented while laser is outputted by the comparation with the theory result and the experiment. The thermal lens can be achieved by experiment at first, then calculated out by the theory analysis. The thermal lens resonance cavity model is set up at first,then the waist of the laser is calculated and the DPL′s far field divergence angle can be measured by the edge of knife method .In the end , using the laser′s spread and lens′ transfer theory , the focus of the thermal lens is known. This is a new method to measure the focus of the thermal lens of the end-pumped Nd∶YAG Laser. The result shows that the theory value is in good agreement with the experiment value. It shows that this method is correct and can be operated easily. Because the experiment is done when the DPL outputs laser , the focus of the thermal lens is gotten according to the circumstance of the laser and the precision is high.