Lake ice thickness changes with regional hydrometeorology and is closely associated with global climate change. We tested the detection of ice thickness using ground penetrating radar (GPR) in the Hongqipao reservoir. Ice crystals, gas bubbles, ice density and ice thickness were also determined by concurrently drilling for validation. During the tests the gas bubble content was high in the upper and low in the bottom, ice density varied with the bubble content, and the ice thickness was not homogeneous. By comparisons between radar detected and in-situ measured ice thicknesses with theoretical analyses of the influence of gas bubble content on the dielectric constant, a three-layer model with snow, granular ice, and columnar ice was established and the transmission speed of radar wave within the ice was determined. Experience reveals that the equivalent dielectric constant can be used to evaluate the wave speed and the values at 1/3 ice cover depth can be used as equivalent values. Besides, the difference between the theoretical transmission time and the real detection time induced by large gas bubbles increases nonlinearly with the theoretical transmission time or ice thickness.