The vibration generated by gear meshing in the encased differential planetary train will be transmitted to the reducer housing. Determining the vibration receiving structure of vibration transmission and analyzing the sensitivity of its parameters can enable researchers to control the parameters with high sensitivity. In this paper, the vibration acceptance structure of encased differential planetary train is analyzed with the time-varying mesh stiffness and error equivalent displacement generated during gear meshing as the internal excitation. Then the first-order trajectory sensitivity model is established based on the state equation of encased differential planetary train. The first-order trajectory sensitivity function amplitude is defined as the first sensitivity indicator S1, and the sensitivity change corresponding to the increase in the percentage of the parameter is defined as the peak sensitivity indicator S2 and the mean sensitivity indicator S3, respectively. On this basis, the sensitivity histogram of each parameter on the response of the vibration receiving mechanism under the three indicators is obtained. The results show that the sensitivity index of the system mesh damping parameter is much larger compared with the mesh stiffness parameter when the parameter unit value is changed, the peak sensitivity index and the mean sensitivity index have high similarity when the parameter percentage is changed.