The spatial and temporal variability of the redundant capacity of special transformers for adjacent buildings in the city brings the feasibility of joint regulation of orderly charging to charging stations built on the basis of the special transformers of buildings. Based on this, a special transformer mutual assistance mode (referred to as mutual assistance mode) and its orderly charging optimization control model are proposed, in which charging station operators combine special transformers across buildings to supply power to clusters of charging stations. First, the interests of operators, special transformer owners and electric vehicle (EV) users are analyzed. And a three-party game regulation mechanism for orderly charging is established. Secondly, in order to quantify the effect and implementation difficulty of the mutual assistance mode, a system of indicators for evaluating the effectiveness of regulation, a evaluation model of the complementarity of joint special transformers and a quantitative model of the difficulty of joint regulation are proposed. And a limit distance evaluation model of joint charging stations is established. Then, comprehensively considering the charging cost of EV users, the rental income of special transformers and the gross profit of operators, an optimization model for the control of orderly charging in the mutual assistance mode is put forward. Finally, the effectiveness of the proposed model is verified.