The present disclosure relates to a spacer configured to space a first member apart from a second member, and the spacer includes a support component and a spacer component. The support component has a body, which has a support surface for the first member to be put on, and the support component further has a first hook element separated from the support surface by a first distance and a second hook element separated from the support surface by a second distance greater than the first distance. The spacer component is used to be placed between the first member and the second member and to space the first member apart from the second member, and the spacer component can be snap-fitted with the first hook element or the second hook element relational to the thickness of the first member. The present disclosure further relates to a connection system including the spacer for a base station antenna. The spacer is advantageous in cost, compact in structure, and easy to install.