Measuring the rotational accuracy of the high precision spindle with microcomputers requires to take the sample at fixed point. Normally, a larger size circular plate is mounted on the spindle. There are many equally-spaced small holes on the circumference. On the two sides of the plate, a photo-diode and a photo sensor diode are fixed. When the diodes and hole lie on the same straight line, the photo diode light falls on the sensor diode. An electric impulse is excited and a sample is taken. This installation is complex and bulky, affecting the rotational accuracy of spindle.This article suggests a new method of sampling. First of all, measure the time of one revolution of the rotating spindle; in the second revolution, determine the time interval of the sampling according to the number of the sample required for one revolution. In the third revolution, the sample is taken according to the time of interval. A measuring system have designed by means of this method and have performed many experiments on the spindle of a gear measuring instrument. The maximum error of the number of sample is one. Compared with the ordinary method, the new method of sampling simplifies the installation to a great extent and increases the accuracy of measurementr considerably.This article also evaluates the accuracy of rotation by means of "least square method" which is worked out by a kit.