Presently, grating encoder is an important way to accurately measure angle, however, the eccentric inaccuracy of assembling adjusting and the vibration of rotating shaft influence grating encoder′s measuring accuracy and then confine the application range of grating encoder. A new method to eliminate the influence of eccentricity and vibration with four reading heads is proposed and the theoretical derivation and simulation results are given out. The conclusion hints that the method which gets moiré fringes signal by four reading heads well-distributed along the circumference of grating is more effective in improving the measuring accuracy than the one that uses double reading heads. Consequently, the new method decreases the requirements of assembling adjusting. The direction and magnitude also can be solved by four reading heads, so the influence of rotating shaft can be eliminated. All the results are useful to deeper research.