In the process of precision compensation of NC machine tools, the traditional 9 line method has such shortages:firstly,principle error is existed in error compensation of the rolling angle, because of difference from the coordinate system of the laser measurement and machine tool manufacturing; secondly, the method has a poor robustness because the measurement noise and the repeatability of the machine tool are included in the measurement; thirdly, the compensation model can only be used for the error compensation, and cannot be used for the precision design and the precision control and process optimization in the process of the machine tool assembly. 9 line method was modified based on error compensation model reconstruction, computer simulation research and empirical analysis. Compared with the experimental results, it is proved that the new method can correct three shortages of the traditional nine line method, and the accuracy of the roll angle error is improved.