The invention discloses a magnetic optical isolator rotation angle measuring method and an optical parameter measuring device. The magnetic optical isolator rotation angle measuring method comprises the steps of: a, providing a laser, a beam expanding mirror, a beam reducing mirror, a polarization analyzer, and a photoelectric sensing device which are arranged along a light path; b, rotating the polarization analyzer to a reference angle display value theta 1, at this time, defining the reading of the photoelectric sensing device as P1; c, placing a polarizer between the laser and the beam expanding mirror, and rotating the polarizer to enable the reading of the photoelectric sensor to be first minimum value P2; d, placing a magnetic optical isolator to be measured between the beam expanding mirror and the beam reducing mirror, adjusting the magnetic optical isolator to enable the reading of the photoelectric sensing device to be maximal, rotating the polarizer to enable the reading of the photoelectric sensing device to be second minimum value Pmin, at this time, defining the rotation angle of the polarizer as theta 2; and e, obtaining the rotation angle theta of the magnetic optical isolator, wherein the theta is the absolute value of the difference value between the theta 1 and the theta 2. The magnetic optical isolator rotation angle measuring method disclosed by the invention has higher measuring accuracy.