The aberration of high-altitude deformable mirror (DM) in multi-conjugate adaptive optics (MCAO) system can lead to the decrease of the image contrast of the extended target and the decline of the closed-loop capability of the MCAO system. However, the aberration of the high-altitude DM in MCAO system is difficult to detect online due to its special conjugate position. The existing MCAO system usually requires regular removal of the high-altitude DM from the system for aberration detection and flattening. Some MCAO systems under construction may integrate interferometers in the optical path of high-altitude DM, but it is very challenging. So far, there is no simple way to detect high-altitude DM aberration online in MCAO system. Therefore, it is necessary to seek an online aberration detection method for high-altitude DM with certain accuracy. For this purpose, a multi-direction wavefront sensor based method is proposed.A multi-direction wavefront sensor can detect aberration information in different sub-area of a high-altitude DM from different line of sights. To retrieve the aberration of the entire high-altitude DM, it is necessary to detect wavefront in multiple directions. By using wavefront stitching method, the wavefronts detected by different line of sight directions should be concatenated in a certain position in order to obtain the complete wavefront information of the high-altitude DM.Numerical simulation and experimental validation are carried out for the MCAO system parameters of the 1-meter new vacuum solar telescope at Yunnan Observatory. The detection capability of the proposed method for high-altitude DM is given (Fig.3). The relationship between the aberration detection accuracy and the conjugate height of deformation mirrors is also given, for the MCAO system of the telescope with 1 m aperture and 1' field of view, the proposed method has a lower error for the detection of deformed mirrors with 2 500-4 000 m conjugate height (Fig.5). Finally, the proposed method is used to online detect the aberration of 313-units DM of NVST MCAO system, and the detection error is 43.6 nm (Fig.9).This article addresses the problem of difficulty in online detection of static aberrations of high-altitude DM in MCAO systems. An online aberration detection method based on pupil multi-direction wavefront sensor is proposed for a dual-conjugate adaptive optics system. Through theoretical and simulation analysis, it is found that the method has detection ability for deformation mirror aberration modes higher than the pupil detection order. However, there is also wavefront stitching error caused by inaccurate measurement of the pupil's Piston mode. In the experiment, the 7×7 sub aperture multi-direction Shack Hartmann wavefront sensor of the NVST MCAO system was used to detect aberration of a 313-units deformable mirror conjugated to 2.8 km. There is very small difference between the results of high-order self-collimated Shack Hartmann and the results of proposed method, which verified the correctness of the proposed method. Without additional detection devices, the proposed method can effectively reduce the static aberration of high-altitude DM, providing good closed-loop initial conditions for MCAO systems.