RF deflecting cavity can be used for bunch length measurement and is designed to diagnose the beam produced by the photocathode electron gun which was built at Tsinghua University for the Thomson scattering experiment.Detailed discussion and calculation for measuring the 3.5 MeV bunch and another with further acceleration to 50 MeV,which is under development,are presented.A standing-wave deflecting cavity working at 2856 MHz is designed and the power feeding system has been planned.