The existing inversion analysis of the mesoscopic mechanical parameters of concrete is generally based on the measured axial strain; however, concrete specimens usually show obvious Poisson effect under loads. So, the mechanical parameters optimized by the combination of measured longitudinal and transverse strain inversion can better reflect the real performance of the material. Here, the uniaxial compression creep Poisson’s ratio test of hydraulic concrete with different loading ages was conducted, and then the elastic parameters of aggregate and mortar were inverted using the combined method of “orthogonal design-neural network-mesoscopic finite element model”, based on the measured longitudinal and transverse strains of the loading and load holding stages. Then, the measured creep variability of concrete was taken as the initial value of the mortar, and the creep variability and the creep Poisson’s ratio of the mortar at the load holding stage were obtained by inversion. Finally, the creep variability of mortar and the creep variability of concrete obtained by inversion were compared and analyzed. The results show that there is a difference between the mechanical parameters of aggregate & mortar and concrete, the elastic Poisson’s ratio of aggregate and mortar obtained from measured strain inversion is between 0.13 and 0.20, and the creep Poisson’s ratio of mortar is about 0.125; under the same loading age, the creep characteristics of mortar are similar to that of concrete and the creep variability of mortar is larger than that of concrete, which is about 2.5 times greater according to this research.