Abstract: The single-user and multi-user ultra-massive multiple-input multiple-output (MIMO) channel measurements at 5.3 GHz band and channel capacity analysis are carried out in this paper. The purpose of the paper is to study the system performance of sixth-generation (6G) ultra-massive MIMO. The transmitter is equipped with single-user or multi-user antennas, the receiver is equipped with the 128×8 ultra-massive MIMO antenna array for channel measurements in urban environments for the uplink. The channel impulse response is obtained by the system calibration and measurement data processing. According to the reciprocity for the uplink and downlink, the channel matrix for the downlink is obtained from the channel matrix for the uplink. Based on mean square error minimization method, the channel capacity optimization problem is transformed into a convex optimization problem. The maximum channel capacity for the downlink can be obtained by optimizing the precoding matrix. The single-user channel capacity in line-of-sight (LoS) environments is smaller than that in non-line-of-sight (NLoS) environments of the downlink. When the number of users increases from single user to 4 users, the multi-user channel capacity of the downlink will increase with the increase of users. In addition, the channel capacity can be improved by using optimization algorithm to design the precoding matrix by the measurement data.