Optical-electronic autocollimation method is commonly used to measure straightness ofprecision guides in engineering application. However, the traditional fixed interval optical-electronicautocollimation method is not suitable for measuring straightness of an air-bearing guide with a longair-bearing bush or a precision straight guide with a long slide-carriage, because the air-bearing bushand the slide-carriage are actually taken as a big bridgeboard bigger than the length of the bridge-board with the reflector, which is about 1/4~1/2 of total length of the measured guide. If straightnessis measured according to the traditional method, only a few points are sampled so that the guidestraightness can not be evaluated fully or accurately. In order to solve the problem, an alterable meas-uring interval method is proposed for straightness measurement based on analyzing the mutual rela-tions and effects among the tilting angle of the reflector, the length of the bridgeboard, the measuringinterval and the straightness of the guide. A straightness calculation model is also developed using themethod, and the errors stemming from the method proposed are introduced in brief. A precisionair-bearing guide with a long air-bearing bush is measured and evaluated using the method proposed,and the actual measurement and evaluation results prove that the method is correct in theory andpractical in operation. The method proposed gives an effective and flexible solution to the straight-ness measurement of the precision guide with long slide-carriage or air-bearing bush in application. Itis an extension of the traditional optical-electronic autocollimation method for straightness measure-ment. Optical-electronic autocollimation method is commonly used to measure straightness ofprecision guides in engineering application. However, the traditional fixed interval optical-electronicautocollimation method is not suitable for measuring straightness of an air-bearing guide with a longair-bearing bush or a precision straight guide with a long slide-carriage, because the air-bearing bushand the slide-carriage are actually taken as a big bridgeboard bigger than the length of the bridge-board with the reflector, which is about 1/4 to 1/2 of the total length of the measured guide. If straightnessis measured according to the traditional method, only a few points are sampled so that the guidestraightness can not be evaluated fully or accurately. In order to solve the problem, an alterable meas-uring interval method is proposed for straightness measurement based on analyzing the mutual rela-tions and effects among the tilting angle of the reflector, the length of the bridgeboard, the measuringint A precisionness-bearing guide with a long air-bearing bush is measured and evaluated using the method proposed, and the actual measurement and evaluation results prove that the the method is correct in theory and practice in operation. The method proposed gives an effective and flexible solution to the straight-ness measurement of the precision guide with long slide-carriage or air-bearing bush in application. Itis an extension of the traditional optical-electronic autocollimation method for straightness measure-ment.