The paper describes the CASIA speech synthesis system entry for Blizzard Challenge 2017. About 6.5 hours of speech data from professionally-produced children’s audiobooks is adopted as the training data for the construction this year. Our synthesis system is built based on the BiLSTM guided unit selection and waveform concatenation approaches by using the provided corpus. Different from our previous system, some improvements about unit selection strategies were made to adapt to different types of the utterance. In this paper, the definitions of the acoustic and the contextual parameters, strategies of candidate unit selection, the calculation of costs based different contexts will be introduced and discussed. Finally, the results of the listening test will be presented.