上海软X射线自由电子激光装置(SXFEL)对束流发射度增长的幅度有着较为严格的限制，传统的光学准直已经不能满足其要求。采用基于束流的准直方法可以实现更为精密的准直，使得直线加速器的准直误差进一步降低，以满足自由电子激光装置的要求。通过对不同条件下束流位置检测器(BPM)测得的数据采用最小二乘法算法进行计算分析，可以计算得到四极磁铁和BPM的准直误差，进而进行束流准直和轨道校正。基于以上原理，计算了各种元件误差对轨道和发射度造成的影响，同时基于Matlab平台设计了控制软件，模拟结果表明轨道偏离量可减少一个数量级。

The SXFEL has stringent restrictions on beam emittance growth, and the traditional optical alignment can no longer meet its requirements, but the beam-based alignment(BBA) method allows more precise alignment, further reducing the linac errors to meet SXFEL requirements. In undulator sections, orbit changes are not only caused by misalignments of quadrupole magnet position, but also the errors of undulator magnet. In order to achieve alignment accuracy over long distance, the quadrupole magnet errors and beam position monitor (BPM) offset are obtained by measuring. BPM data under different conditions and using SVD algorithm for calculation and analysis, With the method above, software based on Matlab has been designed and compared with other software simulation results demonstrate that the orbit offset could be reduced by one order.