This paper aims to design a new 650MHz variable fundamental power coupler. The external quality factor Qe ranges from 1.5×105 to 2×106 to meet the requirements of different current modes of the Beam Test Facility of PAPS (Platform of Advanced Photon Source Technology R&D). This is the first time to develop a variable continuous wave type power coupler in China. In this paper, the RF transmission and field distribution of the coupler are calculated by HFSS three-dimensional electromagnetic software. The external quality factor at different antenna penetration depth is simulated by CST microwave studio. The penetration depth adjusting experiment is carried out by integrating with a 650 MHz 2-cell superconducting cavity. The simulated and measured results of Qe agree well with each other.