In order to overcome the puzzle of being difficult to use GPS to accurately locate oneself in hidden engineering, large shopping malls, underground mines and inner parking lots and other complex environments, this paper explored a sort of indoor trilateration algorithm implementation based on positioning of RSSI using a smart phone. In the paper, it pointed out the challenges of GPS positioning, analyzed the advantages of trilateration algorithm, discussed the principle based on RSSI positioning, and studied on the implementation of positioning algorithm based on smart phone. It built the operation platform and developed an experimental positioning system under the integrated development environment, and the actual test data demonstrated its rationality and feasibility. Experimental results and analysis show that the developed positioning system could meet the some requirements for indoor positioning.