In order to study the mechanical properties of UHV solid composite post insulators, static lateral push test and dynamic property test are carried out. The strain at the bottom of composite post insulators, the force-displacement relationship, and fundamental frequencies before and after the failure are obtained by arranging strain gauges at the bottom of the post insulators, displacement transduces and accelerometers on the top of post insulators, respectively. It’s observed that the fractures on the metal flanges and the bond failure between the post insulator bar and the flange are the most common failure modes, and the fundamental frequency of the post insulators decreased significantly after its failure. The force-displacement relationship of the post insulator under lateral force can be divided into linear elasticity stage and elastic-plasticity stage. Except for the other parameters, the lateral load bearing capacity of the post insulators mainly depends on the height of flanges. Due to the higher bonding region and the better bonding effect, the insulator with a lager flange height will have a greater lateral load bearing capacity when other parameters of the post insulator bar are the same. For composite post insulator without reinforcement rib on the flange, the lateral bearing capacity of its components is different, and the bearing capacity of post insulator bar is higher than that of the flange and the bonding material.