This paper introduces a new impedanceloaded wireless passive sensor. The sensor consists of a SAW device and an external sensor as the impedance load, the SAW device is used for signal transmission. When the sensor is working, the echo wave character will change with the variaty of sensor impadence, so the amplitude and phase of echo wave can correspond to the change of the target detection.We obain the cascaded Pmatrix based on the theory of coupling of mode. Aiming at the two kinds of capacitive and resistive impedance load, the simulation analysis and test of SAW sensors are carried out respectively. The test results show that when the capacitive sensor is used as the load, the phase can be used as a detection reference.