Abstract: On the basis of summing up conventional linearity error testing for guideway, a novel method for testing linearity error of very long guideway by laser tracker is proposed. The basic principle and testing processing of the method are analyzed and researched. A long guideway of 20 m is measured by this method, the linearity error of each direction is 27 m and 25 m respectively. This method enlarges the ability of the laser tracker, it can test the linearity error of long guideway precisely and swiftly, and it can separate the linearity error of two directions, the longth of measurement guideway can reach 70 m, and the testing precision is 0.4 m/m。