Abstract: Waveguide to coaxial line transtion is often used in RF measurement systems. To measure the network parameters of these RF devices with a vector network analyzer and other instruments, the transtion is indispensable to convert the waveguide ports of the RF devices to 50 Ω coaxial line which can directly access the network analyzer. The 648 MHz/WR1500 transformer designed in this paper is mainly used for the measurement of the superconducting linac RF devices at the China Spallation Neutron Source (CSNS) upgrading phase (CSNS-II). In this paper Chebyshev ridge ladder impedance transformation and probe coupling are adopted to complete waveguide to coaxial line transition. The ridge loaded waveguide and discontinuous coaxial line are analyzed respectively, and the optimal size is obtained. The designed transformer has the characteristics of low insertion loss, low VSWR and wide bandwidth to achieve high measurement accuracy. At last, the transition device is tested, and the test results are close to the simulation results, which can meet the requirements of being a measuring device.