A 3D ISAR security imaging experiment at 24GHz is demonstrated with a sparse MIMO array. The MIMO array is an 8Tx/16Rx linear array to achieve real-aperture imaging along the vertical dimension. It is time-switching multiplexed with a low-cost FMCW transceiver working at 22GHz-26GHz. A calibration procedure is proposed to calibrate the channel imbalance across the MIMO array. The experiment is conducted on human moving on a cart, where we take advantage of the linear motion of human to form inverse synthetic aperture along the horizontal dimension. To track the motion of human, a 3D depth camera is used as an auxiliary sensor to capture the rough position of target to aid ISAR imaging. The back projection imaging algorithm is implemented on GPU for quasi-real-time operation. Finally, experiments are conducted with real human with concealed objects and a preliminary automatic object recognition algorithm based on convolutional neural networks are developed and evaluated on real data.