A new method based on the double-beam polarization balance bridge technique for measuring the ultralow total loss, including absorption, scattering, and reflection in ultrahigh antireflection mirror is presented. By rotating the polarizer before and after the insertion of a mirror into one beam out of the Wollaston prism to adjust the intensities of the two beams to be equal, the total loss can be obtained by recording the angle variation of the polarizer. The factors affecting the measurement precision of the technique and the main sources of errors are investigated in detail. The total loss measurement precision of the mirror is 10 ppm and the zero drift is 10 ppm at 632.8 nm.