The ion cyclotron range of frequency (ICRF) heating system of the Experimental Advanced Superconducting Tokamak (EAST) is characterized by high radio-frequency (RF) power up to 12 MW and wide frequency range over 25 to 70 MHz. A high RF power transmission system composed of a liquid impedance matching device, ceramic feedthrough, decoupler, and ICRF heating antenna with four straps has been in operation for some years. In a high-power ICRF experiment, one issue that needs to be solved is the high RF voltage on the coaxial transmission line between the ICRF antenna and the impedance matching device, which is caused by low antenna loading resistance compared to the characteristic impedance of the transmission line. A stub tuner is employed to reduce the RF voltage in the EAST ICRF power transmission system. Two methods to reduce RF voltage using short-circuited and open-circuited stub tuners are introduced in detail. The optimized position and length of the stub tuner are analyzed and calculated to achieve a smaller voltage reduction ratio (VRR) on the transmission line. The test with the stub tuner to reduce the RF voltage of the transmission line is carried out, and a RF VRR of ~0.57 is achieved. The RF voltage on the transmission line is significantly reduced, and the capability of the transmission power is obviously improved. Ohmic losses caused by the surface resistance of the conductor of the coaxial transmission line are also decreased, and the probability of breakdown within the transmission line is reduced under high RF power operation.