In this paper,the change of receiving sensitivity for a MEMS acoustic sensor before and after Parylene-C encapsulation was achieved through finite element simulation, and the conversion coefficient Krt between the receiving sensitivity and vibration amplitude of diaphragm from excitation voltage was derived from the Lump-element model, and then the influence of Parylene-C encapsulation on the device performance is explored. Our experimental results showed that the sensitivity and mean vibration amplitude of the device were reduced to 42% and 48% of the original values, respectively, after encapsulation. The similar reduction percentage in sensitivity and mean vibration amplitude indicated K_rt was independent from the encapsulation. Therefore, the change of device sensitivity can be evaluated by measuring the mean vibration amplitude instead of time-consuming sensitivity measurement, and this method can be used to quickly screen the same batch of devices and improve the efficiency. Moreover, the simulation results show that the sensitivity is reduced to 55% from original value after encapsulating, which is close to the test results of amplitude, and the two confirm each other.