Provided is an evaluation method that provides detailed information on the crosslink densities in sulfur-containing polymer composite materials. The present invention relates to a method of measuring crosslink densities in a sulfur-containing polymer composite material, the method including: a measurement step of irradiating the sulfur-containing polymer composite material with high intensity X-rays and measuring an X-ray absorption spectrum of the composite material while varying the energy of the X-rays; a visualization step of determining the three-dimensional structure of sulfur atoms in the sulfur-containing polymer composite material by the reverse Monte Carlo method from the X-ray absorption spectrum; and a calculation step of calculating, from the three-dimensional structure of sulfur atoms, a crosslink density for each number of sulfur atoms bonded.