Optical Thomson scattering (OTS) diagnostics have been continuously developed on a series of large laser facilities for inertial confinement fusion (ICF) research in China. We review recent progress in the use of OTS diagnostics to study the internal plasma conditions of ICF gas-filled hohlraums. We establish the predictive capability for experiments by calculating the time-resolved Thomson scattering spectra based on the 2D radiation-hydrodynamic code LARED, and we explore the fitting method for the measured spectra. A typical experiment with a simplified cylindrical hohlraum is conducted on a 10 kJ-level laser facility, and the plasma evolution around the laser entrance hole is analyzed. The dynamic effects of the blast wave from the covering membrane and the convergence of shocks on the hohlraum axis are observed, and the experimental results agree well with those of simulations. Another typical experiment with an octahedral spherical hohlraum is conducted on a 100 kJ-level laser facility, and the plasma evolution at the hohlraum center is analyzed. A discrepancy appears between experiment and simulation as the electron temperature rises, indicating the occurrence of nonlocal thermal conduction.