pH value is one of the important parameters for evaluating the silage quality. A wireless pH value monitor device for silage was developed, and it could obtain the pH value of silage at real-time during fermentation. After the calibration of the device, the verification experiments of the actual measurement results were carried out in chopped maize and grass samples. The results 〖JP3〗showed that there is the linear correlation between the measurement result and the accurate pH value, r is 0.997 3 and 0.995 7 (chopped maize, grass sample, pespectirely). The experiments were carried out for monitoring the change of pH value during fermentation in chopped maize and grass silage respectively. The results showed that the original pH value in chopped maize is 5.4, and then the pH value decreased sharply from 5.4 to 4.0 between 0.5~2 d. After that, the pH value decreased slowly and became stable at 3.8 finally; the original pH value in grass is 5.9, and the pH value decreased from 6.0 to 5.2 between 0.8~3 d. Finally, the pH value became stable at 5.1 in 7 d fermentation.