The existence of phase error influences the imaging performance of optical synthetic aperture system and limits the resolution of the imaging system. The relationship between point spread function of binocular system and piston error is analyzed and the conclusion that the relation between point spread function and piston error is irrational with coordinates along the perpendicular bisector of baseline has been deduced and proved. Therefore the effect of position accuracy to error detection is avoided. The distribution characteristic of point spread function on baseline determines the range of piston error and by which a unique value of piston error is obtained. Finally, the piston error detection method of binocular system is extended to the common optical synthetic aperture system. Simulation experiment demonstrates that this method can conquer effect of various noises and the detection accuracy is better than 0.004 times of wavelength.