In this paper, a new SAW gas sensor with three-dimensional nanowire structure is presented. The preparation and modification of nanowire on SAW resonator, the design of sensor signal extraction circuit and the design of alarm detector are described in detail. In the end, a gas alarm detector sample is developed, and the performance index of the sensor is tested with GB simulator, the response threshold of the sensor is 2 mg/m3, and the corresponding time is only 20 s. The results show that the three-dimensional nanowire SAW gas sensor has a significant improvement both in response threshold and in corresponding time compared with the conventional two-dimensional SAW gas sensor.