Abstract: Electron linear accelerator is widely used in the field of nondestructive testing and radiotherapy, its energy can be adjusted in a wide range, and it has advantages in material identification and precise radiotherapy. In this paper, the design and tuning of a S band keV/MeV energy adjustable standing wave accelerating tube were presented. A biperiodic sidecoupled structure was adopted in this tube, and its working frequency is 2 998 MHz. The design high and low beam energy are 450 keV and 6 MeV respectively. Energy switch was proposed to be used in this tube to establish two different kinds of accelerating electric field distributions, so as to meet the requirement of accelerating electron beam to different energy. The challenge is that the keV low energy electron beam is easily lost during the acceleration and transportation due to the phase velocity mismatch between the low energy beam and fixedlength accelerating structure. To reduce the beam loss, it was proposed to keep a low accelerating electric field in the downstream of the accelerating tube for transverse focusing. The beam trajectory and energy spectrum results were calculated by using different field distributions. According to comparation and analysis, the phase velocity distribution and accelerating electric field distribution of this tube were determined and the tube was designed to be composed of two bunching cavities and five accelerating cavities, with a total length of about 330 mm. Then, the equivalent circuit model of the whole tube was established, which is used to analyze the coupling coefficient relationship between adjacent cavities. 3D model of cavity was built to adjust the main size and the whole tube was optimized to obtain the required field distribution for MeV beam acceleration. And the energy switch was designed to be located in the second sidecoupling cavity and its length was simulated to realize the field distribution for keV beam acceleration. Dynamic simulation has been done using the simulated electric field distributions, the beam energy can reach 6 MeV and 450 keV respectively, and the capture efficiencies are 39% and 19%, which meets the design requirements. Finally, the cold test of this tube is carried out after precision machining. After tuning carefully, the electric field distribution before and after the use of the energy switch is basically consistent with the simulation result, which verifies the whole design and development process.