A microwave photonic phase shifter is constructed and tested,which is based on the vector-sum principle and integrates 50∶50 couplers,variable optical delay line and variable optical attenuator.Broadband optical source is used as the carrier to effectively eliminate signal′s instability which is caused by different phase signals′ coupling.The 10 GHz microwave signal′s 350°phase-shift tuning range can be obtained by using the reversal phase characteristic of the Mach-Zehnder LiNbO3 intensity modulator and by tuning the optical power ratio of the carriers between two branches.According to measurements of microwave signal power,the maximal power attenuation appears when the power ratio reaches 1∶1.