A miniaturized 150 kV DC photocathode gun is developed at Peking University to generate electron beam which can be manipulated in temporal and spatial distribution as requirements freely. To measure the bunch length which is an important temporal parameter of the low energy electron beam from the DC photocathode gun, a 1.3 GHz medium-β disk-loaded deflecting cavity is adopted. In this paper we present the design of the deflecting cavity which involves the microwave design including the geometry optimization and the separation of the orthogonal dipole modes as well as the power coupling, the mechanical design including the determination of the cavity wall thickness and the tuning as well as brazing structure, and the thermodynamic analysis. Particle tracking simulation shows that the best resolution of 190 fs can be achieved for the 150 keV electron beam by using the deflecting cavity. Its fabrication is completed and the RF measurements are carried out with a vector network analyzer. It is shown the measured values of the RF physical parameters are in good agreement with the simulation design ones.