To detect the all-sky optical-axis parallelism of the beam launching system,a detection method was proposed and the detection devices were designed. This method works with the visible-detector on the system,and the errors that introduced by detection devices can be measured and wiped out,thus the detecting results are of high precision. Firstly,the principle,the devices and its assembly are introduced; then the relationship between reflector angular variation and the introduced errors is analyzed according to the transformation of coordinates and vector rotation. Finally,an experiment was done to ascertain the deformation of the device,and the introduced errors were wiped out. The parallelism of the system was determined at last. The results show that the deviation of detection results with the actual value is within 2″.