This paper presents an ultrasonic indoor positioning system which is easy to be deployed. Ultrasonic signal whose bandwidth is 20-22 kHz can be transmitted by ordinary tweeters. This paper realizes the time difference of arrival (TDOA) positioning based on ultrasound transmitted by three tweeters. A mobile phone collects ultrasonic signal in this band by microphone. A high accuracy, low-cost indoor positioning can be achieved after signal processing. Signal structure of ultrasonic positioning acoustic source is designed at the transmitter. A synchronization method based on frequency search and FFT search is used at the receiver. A modified SR-1 method is used in positioning scenario based on TDOA. This paper validates that the accuracy of modified SR-1 method is basically the same as the classical CHAN algorithm when three tweeters are used in the scenario. The band of the ultrasonic signal which is used in our scheme is fairly different from those commonly-used scheme whose signal band is 40 kHz. The error is less than 9 cm in every experiment under the condition of receiving ultrasonic signal by an ordinary phone.