The invention provides a method for automatic alignment in grinding of a ball valve. The method comprises the following steps that a high-precision standard ball valve is used for adjusting positions and postures of three eddy current displacement sensors, the relative positions between the sensors and a rotary grinding disk are kept unchanged, a ball value to be ground replaces the standard ballvalue, the longitudinal position of the ball value to be ground is adjusted unitl the sampling average values of the first and the third sensors are the same, so that the automatic alignment is realized. The invention also provides a method for on-line real-time detection in the grinding process of ball valve; after the automatic alignment is realized by the method, the sampling average values ofthe three sensors are worked out in a period of time of non-zero output signal in the second sensor cycle; and the sampling average values of the first and the third sensors are worked out in a periodof time of zero output signal in the second sensor cycle; and finally, the two average values undergo weighted summation to obtain the current size of the ball valve. The method and the device have the advantages of realizing the automatic accurate alignment of the ball valve and the rotary grinding disk, on-line detecting the radius of the ball valve in the grinding process in real time, and realizing the full loop control in the grinding process of the ball valve.