In the detection system based on SAW, the detection precision is affected by the phase ambiguity of the echo phase demodulation. The theory analysis showed that the demodulation method combining the time delay measurement with the phase measurement could uniquely determine the physical quantity to be measured for the delay line SAW sensor. The experiment verified that the proposed method could solve the phase ambiguity problem within a large temperature range and the detection accuracy could be up to 0.5 ℃.