A substrate etching method and a substrate processing device, the substrate etching method includes: S1: placing a substrate to be processed into a reaction chamber; S2: supplying etching gas into the reaction chamber; S3: turning on an excitation power supply to generate plasma in the reaction chamber; S4: turning on a bias power supply to apply bias power to the substrate; S5: turning off the bias power supply, and meanwhile, starting to supply deposition gas into the reaction chamber; S6: stopping supply of the deposition gas into the reaction chamber, and meanwhile, turning on the bias power supply; S7: repeating steps S5-S6, until the etching process is completed. In the whole etching process, the etching operation is always performed, and the deposition operation is performed sometimes. Therefore, during the deposition operation, the plasma in the reaction chamber can etch away at least a part of deposited polymers formed by the deposition operation on a sidewall of an etched section, so that the sidewall of the etched section of the substrate is smooth.