Large-scale MIMO systems need multiple antennas in a limited space, which will inevitably introduce coupling and will affect the performance of MIMO systems. This paper studies the Water-Pouring capacity of MIMO systems based on S-parameters. Through the S-parameter network model, the Water-Pouring Principle is introduced on the power distribution of the transmission end, and the capacity closure formula under different matching conditions is derived. The capacity under different matching conditions of the receiver is simulated by Matlab, including: no coupling, self-impedance matching, optimal matching. The results show that the introduction of the Water-Pouring Principle maximizes the power of the transmission end and the system performance is greatly improved under the condition of optimal conjugate matching at the receiving end.