Tetragonal tungsten-bronze structure Sr1.8Ca0.2NaNb₅O₁₅ (SC NN) thin films have been grown on Si(001) single crystal substrates via sol-gel and layer-by-layer annealing method.The crystallinity,surface morphology and cross-sectional microstructure of thin films are investigated by X-ray di ffraction (XRD),atomic force microscopy (AFM) and scanning electron microscopy (SEM),respectively.The resul ts indicate that the highly (001)-oriented SCNN thin films are successfully obtained with low thicknesses (about 150nm,less than 10spinning layers).However,with the increase of film thickness (about 300nm,mor e than 20spinning layers),the trend of polycrystallinity of SCNN film is stren gthened and its preferred (001) orientation is also restrained.The competi tion of homogeneous nucleation insid e the films and heterogeneous nucleation at the substrate interfaces is suggested to account for the change of crystallinity of SCNN films with increasing thickness.The improved simplex method is utilized to determine the refractive indices and thickness of SCNN films by fitting the measured reflectance curves with visi ble reflection spectrophotometer in the wavelength range of 400－780nm.The Sellmeier equation is selected as dispersion model and the double-layer film system model is constructed by considering the refractive i nhomogeneity (as an interfacial layer).The results suggest a great agreement between the measured and calcula ted reflectance curves.The fitting results of film thicknesses are also proved by the SEM patterns.The variation of refractive index keeps consistent with that of crystallinity of SCNN thin films with increasing thickne sses.