The function of laser pulse energy to the radius of the crater produced by focused laser beam on sample′s surface is derived theoretically under the condition that the fluency distribution of the focused laser beam on the focal plane is Gaussian shape. An aluminum foil, an aluminum reflection mirror and a piece of thermo-sensitive photographic paper are used as samples and the radii of the craters produced by laser beam with different pulse energies on sample′s surfaces are measured with a microscope. The spot size of focused laser beam on the focal plane and the laser-ablation threshold of the sample are measured simultaneously by curve fitting. The laser-ablation thresholds of aluminum and thermo-sensitive photographic paper for 532 nm laser pulse with 15 ns pulse width are determined to be 2.5 J/cm2 and 0.25 J/cm2, respectively. Measurement errors for both spot size and laser-ablation threshold are about ±10%. It is demonstrated that the approach introduced here is able to determine spot size of the focused laser beam on the focal plane and the laser-ablation threshold of the sample conveniently and simultaneously. It is able to be used in researches on laser interaction with solid materials and to evaluate the characterization of a focused laser beam.