A method of in-situ measurement of target laser spot-size was provided.Based on the transverse light intensity distribution of the Gaussian beam,the relationship among laser ablation radius and irradiation laser energy,spot-size and ablation threshold was established.The simulation analysis found that the size of the irradiated laser spot has a great influence on the change curve of the radius of the ablation spot with irradiation energy.For laser with a pulse width of 2 ms and a wavelength of 1064nm,radius of ablation spot of photographic paper under different energy laser irradiation was measured,and the spot-size and sample ablation threshold at the target surface were obtained by fitting measured data using the derived relationship.At the same time,the spot-size and sample ablation threshold at different locations were also measured,and the Gaussian beam waist position and the spot size effect of sample ablation threshold were verified.The research results show that the technical results are reliable and the method is simple and efficient.This technology can help to easily and conveniently measure the size of the target spot in basic applications such as high-energy laser and solid matter interaction and laser processing applications.