By analyzing two operational modes of DC systems， the influence of the power of multi-infeed direct current（MIDC） systems on the loss of the inverter-side AC system is discussed in this paper. A method is presented to calculate the loss sensitivity at the two operational modes. One operation mode is that the total active power of MIDC systems is kept constant， and the other is that the active power of each line in MIDC systems is independent of each other. The sensitivity needs to be modified with load equilibrium factor in the latter case. At the first case， an effective algorithm for adjusting the active power of MIDC systems to reduce the loss of inverter-side AC system is presented. Numerical tests in IEEE 118-bus system show that the proposed algorithm is effective and the average reduced loss of 118-bus system with 3-infeed DC system is 0.97%.