Film thickness measurement can be realized using white light interferometry, but it is challenging to guarantee high precision in a large range of thicknesses. Based on scanning white light interferometry, we propose a spectral-temporal demodulation scheme for large-range thickness measurement. The demodulation process remains unchanged for either coatings or substrate-free films, while some adjustments are made according to the estimated optical thickness. Experiments show that the single-point repeatabilities for 500 nm ${\mathrm{SiO}}_2$ coating and 68 µm substrate-free Si film are no more than 0.70 nm and 1.22 nm, respectively. This method can be further developed for simultaneous measurement of surface profile and film thickness.