Various substrate materials exert distinct influences on the crystallinity and stress of polycrystalline diamond films. This study investigates the impact of three substrates （W， Si， and Mo） on the stress and crystallinity of polycrystalline diamond films synthesized using microwave plasma chemical vapor deposition （MPCVD）. Initially， to mitigate edge effects， numerical simulations and optical emission spectroscopy （OES） were employed to optimize and validate that the optimal base height for diamond film deposition is 16 mm. Growing polycrystalline diamond films under the same process conditions， the crystallinity and quality of diamond films deposited on these three substrates were evaluated via Raman spectroscopy and scanning electron microscopy （SEM）. It is found that all diamond films exhibit a （111） orientation， with the W substrate yielding superior crystallinity uniformity. X-ray diffraction （XRD） stress analysis reveals that the polycrystalline diamond film deposited on the W substrate demonstrated uniform stress distribution from center to edge， with minimal stress levels. Consequently， a polycrystalline diamond film with a diameter of 50.8 mm， thickness of 0.6 mm， and deposition time of 200 h is successfully deposited on the W substrate， featuring few impurity defects， no cracks， and a full width at half maximum（FWHM） ranging from 8.156 cm^-1 to 8.715 cm^-1 from center to edge. Transmission electron microscopy （TEM） detected a d-spacing of 0.206 nm for the （111） crystal planes. These findings indicate that the W substrate is more suitable for depositing polycrystalline diamond films intended for optical， thermal， and electronic applications compared to Si and Mo substrates. This research provides valuable insights for the deposition of large-size， low-stress， high-crystallinity polycrystalline diamond films and the application of microwave windows in nuclear fusion tokamak devices.