Photon statistical properties of coherent field and thermal field are experimentally studied by means of direct photon counting using only one single-photon-counting module (SPCM) operating in Geiger mode. The second-order degree of coherence as affected by photon counting rate and selected resolution time is investigated systematically. By taking into account all the experimental factors which can influence the g(2), the second-order degree of coherence of an unknown optical field can be determined quickly and simply via single SPCM when choosing proper detecting conditions. In our experiment, when the counting rate is about 109 kc/s and the resolution time varies from 28 ns to 212 ns, the measured results can reliably reflect the different second-order degree of coherence of coherent and thermal fields.