A simply method to measure the thermal focal length of LD end pumped solid state lasers with stable resonators was presented. By scanning the slit across the field with expression of multi mode Gaussian distribution behind the output mirror of lasers, the radius and the M 2 factor describing the quality of beam were measured. The waist of the multi mode field and the TEM 00 mode at the same parameters of lasers could be deduced according to the law of multi mode Gassian beam propagation. Based on the standard matrix theory of stable resonator, the thermal focus length of the active medium was easily obtained. The thermal focal length of a LD end pumped Nd∶YVO 4 laser was measured and the experimental results were agreed with the prediction of the theory.