The ultra-high precision anti-backlash planetary reducer has an angle transmission accuracy of up to 7 arc seconds and a positioning accuracy of up to 5 arc seconds, which is much higher than the accuracy level of current harmonic reducers and RV reducers. Its torsional rigidity and main bearing overturning rigidity are much stronger than those of RV reducers, and the service life is longer than that of traditional harmonic reducers. Ultra-high precision anti-backlash planetary reducers give robots and machine tools better performance. They push the accuracy limit of ultra-high rigidity reducers to 15 arc seconds, filling the market gap and making ultra-high precision reducers available for high-end industrial machines.