This paper introduces an on-line photoelectrical measuring technology for rotor speed and its direction, which uses quadrant Si-photoelectric cell as the detector. The rotor speed is calculated by measuring the signal frequency and periods together, so the measuring range is very large and the precision is high. Because the subtraction processes is adopted, the system can get over the noise and outside disturbance signals effectively, and the pass-zero processing method can get over the effect of the optics facular intensity changes.