The tiny resonant mass and high Q performance of phononic crystal (PC) point defect make it an excellent candidate for high sensitivity and resolution mass sensors, but its mass sensitivity is highly dependent on the mass positions. In this paper, we fabricate the siliconbased PC point defect structures and establish a springmass model to evaluate the mass sensitivities of the defect modes. The experimental results agree well with the model and the highest mass sensitivity of 9.1 Hz/ng is obtained. Furthermore, we propose a method utilizing the diverse sensitivity distributions of the defect modes to decouple the mass and position simultaneously and demonstrate it in simulation. The results show that the relative errors are 2.6% and 0.43% respectively with 3 and 8 modes for mass measurement.