Abstract: Unified mathematics basic of indoor and outdoor map is an important foundation for spatial data share and use between indoor and outdoor, and an important guarantee of the indoor and outdoor integrated LBS, which plays an important role in safety management and emergency response in complex indoor and underground space. Indoor map usually uses user-defined local plane coordinate system, which results in contradictions in terms of mathematics basic between indoor and outdoor map, and also among different building's indoor maps. Taking geographical coordinate system as the unified mathematics basic adopted by indoor map spatial data, this paper studied the method for transforming indoor map from local plane coordinate system into geographical coordinate system. Through analysis the applicability of traditional map mathematical transformation methods, this paper proposed a method for indoor map unified mathematical transformation based on transition projecting plane. Choosing a residential area's survey data to perform a transformation experiment, the results show that the method proposed in this paper can improve the accuracy of indoor map mathematical transformation efficiently in contrast to traditional methods.