Abstract: Using pulse laser deposition（PLD） technology,Ba_0.4Sr_0.6TiO₃ ferroelectric films of approximately epitaxial growth were fabricated through a series of experiments.Effects of substrates on the quality of epitaxially grown Ba₉0.4) Sr_0.6TiO₃ ferroelectric films were researched.Though the mismatch between strontium titanate substrate（STO） and Ba_0.4Sr_0.6TiO₃ ferroelectric film was the smallest,yet the full wave at half maximum（FWHM） of rocking curves and atomic force microscope（AEM） images indicate that the quality of epitaxially grown Ba_0.4Sr_0.6TiO₃ ferroelectric films on lanthanum strontium aluminate tantalate （LAST） substrates was even better than grown on strontium titanate substrates.It is not the traditional view that the smaller lattice mismatch is the better.This is the main reason that the small mismatch will go against stress relief.