Abstract: The existing readers matching SAW（Surface Acoustic Wave, SAW） tags usually use a non-coherent superheterodyne structure, and generate the excitation pulse signal by a switch. Because of the randomness of the switch closing time and the initial phase of the local oscillator signal, the echo phase measured in this scheme is an uncertain value, which cannot establish a functional relationship with the measurement like distance. In this paper, we design a kind of phasecoherent reader, which divides the signal generated by the signal source into two signals in the same phase with the power divider in the transmitting link, and introduces one of them into the lower mixer of the receiving link as the local oscillator signal, so that the echo phase can be a definite value. The actual reader circuit is made, the transmitting and receiving links are tested, and the experiment is carried out with the SAW tag. The comparison with the measurement results of the vector network analyzer shows the effectiveness of the reader.