This paper presents a method of phase measurement for UWB arbitrary signals, which is based on the spectrum analyzer to measure the amplitudes of two different signals, and then based on the power synthesizer to synthesize the amplitudes of signals, using the principle of vector synthesis, carries out mathematical analysis in the frequency domain, and establishes a method to measure the phase of arbitrary signals. According to the basic principle design, a phase measurement system is established, and the correctness and feasibility of the method are verified by simulation and experiment. In addition, the influence of the parameters such as reflection and isolation of the power synthesizer on the signal phase measurement is discussed. This method is suitable for measuring the phase of signals from DC to tens of GHz. It can be applied to the analysis of electromagnetic interference and signal integrity. It makes up for the shortcomings of the existing phase measurement methods.