This paper explores the intellectual sequence of Daśabhūmi-śāstra School's theory of Buddha-nature with Thusness(Tatathat), true unsullied consciousness and other key concepts by investigating the texts of Daśabhūmi-śāstra School in Dunhuang documents as well as the works of Huiyuan(523—592), an eminent monk in Jingying Temple in the Sui Dynasty(581—619). The main aspects of this theory are as follows: First, it interprets Buddha-nature through the concepts of Thusness and the embodiment of truth and law(dharmakāya) and discusses the doctrine of existence and cultivation methodology of Buddha-nature with the concepts of ti(substance) and yong(function), yin(concealing) and xian(revealing); second, it interprets Buddha-nature with the concepts of Buddha's spirit, true Buddha-mind, and true unsullied consciousness, highlighting the subject state of Buddha-nature. This paper also explores the causality between Buddha-nature and Buddha-embodiment(buddhakāya), showing the identical Buddha-nature in both reincarnation and nirvana. In this way, truth, substance and the subject are unified.