In this paper, the structure and testing principle of vibrating wire pressure cell are presented firstly. Then, combined with the practice in the south anchorage of Jiangyin Yangtze Bridge, the feasibility and difficulty to monitor stress of rigid interface using vibrating wire pressure cell are analyzed. The expression of rating factor is deduced. And on the base of FEM, the method to modify observation data is introduced. Finally, the relationship between modification factor and medium rigidity is deduced. Study results indicate that, while monitoring the stress of rigid interface, the observed values are less than the actual stress values. Even, they are approximately one tenth of the theoretical value. The reason is that the real load condition of the pressure cell is obviously different from the rating load condition. The rigidity of medium affects observation results.