RV is known as the title of "home" on wheels, having two big functions following both "room" and "car". But its attribute is car, and is a kind of portable car having the basic facilities of home. The common types of RV are self-propelled, pull-type, mobile villa and cross-country. In the paper, starting from the actual needs of people, the outward extension part of the self-propelled A-RV has been converted into a balcony in order to make the person having a large living room, kitchen or bedroom and let people travel at the same time find the feeling of the real home. But in this paper, the hydraulic parts and working principle of the extend balcony in the central body have been designed and calculated. According to the results, the balcony can make the internal space extension of 4 square meters, the working principle was reasonable, the calculation of hydraulic driving part conformed to the design requirements.