The invention belongs to the field of machining, and particularly relates to an ultraprecision machining method for a gem substrate based on abrasive material polishing pads. The diamond solidified abrasive material polishing pad with the granularity not larger than 14 microns is used for conducting rough polishing machining on an LBO crystal, in the polishing machining process, the polishing pressure is controlled to range from 50 g/cm<2> to 600 g/cm<2>, the rotation speed of a polishing machine is controlled to range from 10 rpm to 200 rpm, and the PH value of polishing liquid used for polishing is controlled to range from 2 to 6. The temperature of the polishing liquid ranges from 20 DEG C to 30 DEG C, and rough polishing of the LBO crystal is finished. Then, the cerium oxide solidified abrasive material polishing pad with the granularity not larger than 3 microns is used for conducting fine polishing machining on the LBO crystal obtained through rough machining, in the fine polishing machining process, the polishing pressure is controlled to range from 50 g/cm<2> to 600 g/cm<2>, the polishing rotation speed is controlled to range from 10 rpm to 200 rpm, meanwhile, the pH value of polishing liquid used for polishing is adjusted to range from 2 to 6, and the temperature of the polishing liquid is controlled to range from 20 DEG C to 30 DEG C till the surface quality meets the set requirement. According to the ultraprecision machining method, the machining efficiency is high, the finished product qualification rate is high, and environment pollution cannot be caused.