Using multi-layer composite Ta/TbDyFe/Cr film as the sensitive material, a surface acoustic wave(SAW) magnetic field sensor with high sensitivity and long-term stability is optimized. We developed a delay line SAW device with a center frequency of 150 MHz, using radio frequency(RF) reactive magnetron sputtering to deposit Ta/TbDyFe/Cr film on the delay line of the SAW device. Then, we integrated the coated device and the uncoated device into a differential oscillation circuit. Under the action of magnetic field, the magnetostrictive effect produced by TbDyFe film causes the propagation speed of SAW to change, then the corresponding differential oscillation frequency change is used to evaluate changes in magnetic fields. In order to further improve the long-term stability of the sensor, a 30 nm Cr transition layer and a 10 nm Ta protective layer were introduced. The experimental results show that the developed magnetic field sensor has long-term stability and high sensitivity (6.55kHz/mT).