This paper proposes constant voltage design based on K-inverter for cooperative inductive power transfer (IPT) where a nearby receiver picks up power and simultaneously cooperates in relaying the signal toward another distant receiver. In a cooperative IPT system, wireless power is fundamentally transferred to the nearby receiver via one K-inverter and to the distant receiver via two K-inverters. By adding one more K-inverter to the nearby receiver, our design is among the simplest methods as it delivers constant output voltage to each receiver via two K-inverters only. Experimental results verify that the proposed cooperative IPT system can stabilize two output voltages against the load variations while attaining high RF-RF efficiency of 90%.